Merge bitcoin/bitcoin#17487: coins: allow write to disk without cache drop

1d7935b45ac61791399989effc18aece8b368fbb test: add test for coins view flush behavior using Sync() (James O'Beirne)
2c3cbd6c007a588e667751024027462268626fdb test: add use of Sync() to coins tests (James O'Beirne)
6d8affca96c7a34f5f104c5a3122e7420ffc083c test: refactor: clarify the coins simulation (James O'Beirne)
79cedc36afe2e72e42839d861734d73d545d21b8 coins: add Sync() method to allow flush without cacheCoins drop (James O'Beirne)

Pull request description:

  This is part of the [assumeutxo project](https://github.com/bitcoin/bitcoin/projects/11):

  Parent PR: #15606
  Issue: #15605
  Specification: https://github.com/jamesob/assumeutxo-docs/tree/master/proposal

  ---

  In certain circumstances, we may want to flush chainstate data to disk without
  emptying `cacheCoins`, which affects performance. UTXO snapshot
  activation is one such case, as we populate `cacheCoins` with the snapshot
  contents and want to persist immediately afterwards but also enter IBD.

  See also #15265, which makes the case that under normal operation a
  flush-without-erase doesn't necessarily add much benefit. I open this PR
  even in light of the previous discussion because (i) flush-without-erase
  almost certainly provides benefit in the case of snapshot activation (especially
  on spinning disk hardware) and (ii) this diff is fairly small and gives us convenient
  options for more granular cache management without changing existing policy.

  See also #15218.

ACKs for top commit:
  sipa:
    ACK 1d7935b45ac61791399989effc18aece8b368fbb
  achow101:
    ACK 1d7935b45ac61791399989effc18aece8b368fbb
  Sjors:
    tACK 1d7935b45ac61791399989effc18aece8b368fbb

Tree-SHA512: 897583963e98661767d2d09c9a22f6019da24125558cd88770bfe2d017d924f23a9075b729e4b1febdec5b0709a38e8fa1ef94d62aa88650556b06cb4826c845

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