203f212 Reduce default number of blocks to check at startup (Pieter Wuille)

Github-Pull: #8611
Rebased-From: 203f212

203f212 Reduce default number of blocks to check at startup (Pieter Wuille)

203f212 Reduce default number of blocks to check at startup (Pieter Wuille)

bitcoin#8611

203f212 Reduce default number of blocks to check at startup (Pieter Wuille)

73d7288 [Doc] Mention reindex-chainstate in the release notes (random-zebra)
c45c0ec Skip MoneySupply update during reindex/reindex-chainstate (random-zebra)
efe193f Scan for better chains in ThreadImport (random-zebra)
0aceea3 Make ProcessNewBlock dbp const and update comment (Pieter Wuille)
8255e82 [Cleanup] Remove fVerifyingBlocks global flag (random-zebra)
30c7aed Add -reindex-chainstate that does not rebuild block index (random-zebra)
79d5cb2 Verify DB with original default check-level 3 (random-zebra)
ed96859 Reduce default number of blocks to check at startup (Pieter Wuille)
d4516fa Log/report in 10% steps during VerifyDB (Jonas Schnelli)
cce2c9e doc: Mention dbcache increase in release notes (random-zebra)
11dc022 [GUI] Update default db cache size in the options model (random-zebra)
9b7f764 Bump `-dbcache` default to 300MiB (Wladimir J. van der Laan)

Pull request description:

  This was attempted already in #1864 and #1907.
  Third time's a charm.

  Now the speed is acceptable and (unlike what was happening in #1907) the process can be interrupted.
  Further, as it is performed in ThreadImport, the GUI is open and functional during the chainstate reindex, instead of being seemingly stuck at the splash screen.

  This PR also includes a few improvements coming from bitcoin#10919, bitcoin#8273, bitcoin#8136 and bitcoin#8611.

  Here's a quick comparison of the data I had testing here (empty wallet with GUI, empty pivx.conf / default values).
  The reindexes have been performed with 0 network connections.

  <table>
  <tr>
  <th>Mainnet (height=2847011)</th><th>Master</th><th>this PR</th>
  </tr><tr>
  <td>online sync</td><td>8 hr 13 min</td><td>6 hr 42 min</td>
  </tr><tr>
  <td>-reindex (offline)</td><td>7 hr 14 min</td><td>6 hr 21 min</td>
  </tr><tr>
  <td>-reindex-chainstate (offline)</td><td>N/A</td><td>6 hr 13 min</td>
  </tr><tr>
  <td>startup (synced node)</td><td>100 sec</td><td>83 sec</td>
  </tr><tr>
  <td>RAM (synced node)</td><td>1.65 GiB</td><td>1.5 GiB</td>
  </tr><tr>
  <td>utxo_cache (synced node)</td><td>28.9MiB</td><td>39.5MiB</td>
  </tr>
  </table>

ACKs for top commit:
  furszy:
    re-ACK 73d7288, tested again.

Tree-SHA512: fb828d89692ccb6105eae5411b70a3c7a129d98d3101121cf604171de1b818a8c68b3771cc69fcf0a04642fe1327ee7ce360a72ecd7ef019b1614bd83c081de5