(When trying to rebase and clean up the commits in #923 ; the PR automatically closed during the cleanup, and I can't seem to get it back open.)

Problem:
A problem was introduced with #518 when a UTXO smaller than 10% of the threshold is added to the collection of UTXOs to combine, and that UTXO is enough to increase the total amount being combined above the threshold, or when the total amount of dust in the wallet exceeds the threshold by less than 10%.

What occurs is two fold. First, the nTotalRewardsValue > nAutoCombineThreshold will break it out of the for loop; but the "safety margin" will split it up into two UTXOs; one within 10% of the threshold, and then one for the change. When the wallet comes back through on it's dust collection, it can pick up those two UTXOs and repeat until the fees whittle the two combined transactions to a total below the threshold.

If there is another UTXO to add, which takes the logic far enough above the threshold to remain above the threshold after the 10% reduction; the check in the loop is functional. However there is still one other problem case: If the total amount being collected falls into the "within 10% of the threshold" situation, and the for loop can't make another pass... we exit the for loop normally, and find our way into the zero fee check. However the zero fee check sees we are over the threshold, but not that the transaction amount will be over the threshold. So we need to account for the 10% there as well, by using the actual amount (vsecSend[0].second) rather than nTotalRewardsValue to determine if we should continue only if free.

Even with this fix, AutoCombineRewards was still hardly usable, as wallet scans on wallets with many active addresses was consuming significant resources when running AutoCombineDust() on every block. In order to make this feature more useful and usable, further features were folded in; The ability to configure how often to run the scan, and to configure it to combine as much as possible when it executes.

Improvement 1: Max Threshold:
Users trying to clean up wallets that have collected a lot of small transactions may find it more convenient to combine as much as possible with one sweep. Rather than guessing what amount would be best, and having to do multiple passes; setting a threshold of "0" will autocombine up to the max it is capable of. This is also even more useful for users that have several transactions across each of many addresses,, as they would need to get each wallet down to a very few transactions before sweeping the funds into a single large transaction across many input addresses. This allows for autocombine to quickly and easily combine to very few UTXOs in each wallet, and allowing a sweep across many wallet addresses to be much more convenient. This would function very much like a "defragment" routine.

Improvement 2: Frequency:
An old stale "fix" was floating around in internet, implementing an AutoCombineRewardsThresholdTime concept; I was unable to locate the originator. This implementation had allowed you to configure the number of minutes between dust collection. That old algorithm was commented out and replaced with a 5 second wait. This concept also was flawed, in that it was going to be doing a sleep within the ProcessNewBlock() code. The first attempt was abandoned because it likely was blocking ProcessNewBlock for 15 minutes at a time, and no way to not block, if using the feature, for under a minute. The workaround (5 seconds) still wasn't desirable, as it would still lock up block ProcessNewBlock's execution for 6 seconds.

This new method changes that design from ThresholdTime, to Block Frequency, and defaults to 15 blocks. Time can be adjusted per implementation, to get a desired minute time based on the block frequency of the coin parameters. If AutoCombineDust is enabled, it will only check and attempt to combine dust if the block height is a multiple of the Block Frequency. e.g. if set to 10, then every 10th block it will be executed. 100; then every 100th block.

This can now be tailored by the user, based on their desired dust cleanup threshold, and their expectation of frequency that they will need to clean.

Improvement 3: One Shot:
The concept of a "one shot" dust cleanup was added when the block frequency is set to "0". This will run the automatic rewards combining on the next block, and then it will disable itself until the next time the wallet is started up. This feature assumes that someone would be running their wallet infrequently, and therefore is likely to want the one shot to run when they start the wallet again. As such, the enabled=true and frequency=0 is saved in the database, but after it runs, it will be shut off for the duration of the wallet run.

To run it for a one time only execution, the user would have to autocombinerewards false after the one shot is executed.

The feature, in it's entirety:

• Defaults to false
• When "true", requires a threshold (in coin count). Threshold of "0" will combine up to the maximum transaction size.
• Block frequency defaults to every 15 blocks. This can changed to every block (resource intensive) or however many blocks one expects their threshold to hit.
• Block frequency of "0" will run the sweep on the next block, and then turn it off until they run it again, or restart their wallet.
• To prevent the one shot to run on startup, they simply need to turn it off after the sweep is complete (when getautocombineinfo returns "on startup" rather than "on next block")

Examples:

Basic configuration for frequency use

autocombinerewards true 5 250

{
  "enabled": "on",
  "threshold": 5,
  "frequency": 250
  "comment": "Enabled for repeat on frequency"
}

Will run every time the block count is a multiple of 250, and combine if it finds multiple UTXOs. It will collect inputs until the total exceeds 5.5 PIVX. If the total is below the threshold + 10%, it will only submit the transaction if the fee is considered free.

Configuring for frequency use, maximum combine

autocombinerewards true 0 250

{
  "enabled": "on",
  "threshold": 0,
  "frequency": 250
  "comment": "Enabled for maximum combine, repeat on frequency"
}

Will run every time the block count is a multiple of 250, and combine if it finds multiple UTXOs. It will collect as many inputs as possible, up to the maximum bytes allowed per transaction (100kb).

Configuring One Shot

autocombinerewards true 5 0

{
  "enabled": true,
  "threshold": 5,
  "frequency": 0
  "comment": "Enabled for one shot on next block"
}

Before the run

getautocombineinfo

{
  "enabled": true,
  "threshold": 5,
  "frequency": 0
  "comment": "Enabled for one shot on next block"
}

After the run

getautocombineinfo

{
  "enabled": false,
  "threshold": 5,
  "frequency": 0
  "comment": "Enabled for one shot on startup"
}

Please Note: The underlying functionality has been tested on two altcoins, and the RPC logic has been tested standalone on PIVX testnet.