2105947 Implement helper class for CTxMemPoolEntry constructor (Alex Morcos)
1cef905 Make -checkmempool=1 not fail through int32 overflow (Pieter Wuille)
0f72ff2 Support -checkmempool=N, which runs checks on average once every N transactions (Pieter Wuille)
89483d0 [Bug] Make operator() a const function in CompareTxMemPoolEntryByX (random-zebra)
a50ad77 Lower default policy limits (random-zebra)
03f7152 fix locking issue with new mempool limiting (random-zebra)
1598961 Fix stale comment in CTxMemPool::TrimToSize. (random-zebra)
98d0d68 Undo GetMinFee-requires-extra-call-to-hit-0 (random-zebra)
6ad6ee6 Add reasonable test case for mempool trimming (random-zebra)
8dcbb7e Only call TrimToSize once per reorg/blocks disconnect (random-zebra)
c20cd38 Implement on-the-fly mempool size limitation. (random-zebra)
aee2e17 Print mempool size in KB when adding txn (random-zebra)
f7c85fd Add CFeeRate += operator (random-zebra)
5bd2a00 Track (and define) ::minRelayTxFee in CTxMemPool (random-zebra)
0b50f6c Add Mempool Expire function to remove old transactions (random-zebra)
d26f5e0 Fix calling mempool directly, instead of pool, in ATMP (random-zebra)
fc5eddb Reverse the sort on the mempool's feerate index (random-zebra)
0ce1df0 [BUG] Fix CTxMemPool::check excluding zerocoins from children checks (random-zebra)
1f7bd52 Track transaction packages in CTxMemPoolEntry (random-zebra)
1fd406b TxMemPool: Change mapTx to a boost::multi_index_container (random-zebra)

Pull request description:

  built on top of
  - [x] #1645

  This PR pulls some updates from upstream in the mempool area, adding the required adjustments for legacy zerocoin txes and updating the functional test suite.

  Specifically, here we:
  - track mempool descendants (in-mempool transactions that depend on other mempool transactions)
  - turn `mapTx` into a `boost::multi_index_container` that sorts the mempool on 3 criteria:
    - transaction hash
    - fee rate
    - time in the mempool
  - Add a max size for the mempool (throwing away the cheapest txs and bumping the min relay fee, when full)
  - Implement on-the-fly mempool size limit with the flag `-maxmempool`
  - Implement `-checkmempool=N` to customize the frequency of the mempool check
  - Implement helper for `CTxMemPoolEntry` for the unit tests.

  Backports:

  - bitcoin#6654
  - bitcoin#6722 [`*`]
  - bitcoin#6889
  - bitcoin#6771
  - bitcoin#6776
  - bitcoin#6896
  - bitcoin#7020

  [`*`] excluding bitcoin@9e93640 as our default minimum tx fee rate of 10k satoshis is only 0,00003 USD at the time of writing.

ACKs for top commit:
  Fuzzbawls:
    utACK 2105947
  furszy:
    Re utACK 2105947 and merging this nice upgrade :) .

Tree-SHA512: 51a7d75bd52f7646d461252c78f0dd9d7e8b5c1c66c22944120bfe293b28f5d48135de339ebf3d8a5b4c61ca5452383ed1b10c417be06dc4a335ac645842ea14

Document the changes introduced in:

- PIVX-Project#1549: Nuke zPIV from the GUI
- PIVX-Project#1586: Minimum value for stake split threshold
- PIVX-Project#1633: Bitcoin 0.12-0.14 serialization improvements
- PIVX-Project#1645: Implement accurate memory accounting for mempool
- PIVX-Project#1647: MemPool package tracking and limits
- PIVX-Project#1650: Benchmarking Framework
- PIVX-Project#1688: TopBar navigation (sync/peers)

4819ee7 [Doc] Add/Update some release notes for 4.2 (random-zebra)

Pull request description:

  Document the changes introduced in:

  - #1549: Nuke zPIV from the GUI
  - #1586: Minimum value for stake split threshold
  - #1633: Bitcoin 0.12-0.14 serialization improvements
  - #1645: Implement accurate memory accounting for mempool
  - #1647: MemPool package tracking and limits
  - #1650: Benchmarking Framework
  - #1688: TopBar navigation (sync/peers)

ACKs for top commit:
  furszy:
    utACK 4819ee7
  Fuzzbawls:
    ACK 4819ee7

Tree-SHA512: 62ad949ea26a2f877ef0b40ec86616cc8105f81e1fcd380c8162cd93af04a46f1093f878c0668408654f198a0059b240798b83af3bf1d5e6c1c1d8611276a325