Merge #19806: validation: UTXO snapshot activation

1afc0e4aa1b910991d4f8a77d74e2197f370987c doc: remove potentially confusing ChainstateManager comment (James O'Beirne)
769a1ef9fdc9c372f5bbe91d1961cabd60bc1895 test: Add tests with maleated snapshot data (Fabian Jahr)
4d8de04f32736199e4b41a14a2d29b1a4d0a15d4 tests: add snapshot activation test (James O'Beirne)
31d225274ff1a4b245aea0a69f0e5224b0e64ca2 tests: add deterministic chain generation unittest fixture (James O'Beirne)
6606a4f8c616cf256537c3bfbdade9b43c51b4f5 move-onlyish: break out CreateUTXOSnapshot from dumptxoutset (James O'Beirne)
ad949ba449ff2115e3d22c71f5b6509f11112098 txdb: don't reset during in-memory cache resize (James O'Beirne)
f6e2da5fb7c6406c37612c838c998078ea8d2252 simplify ChainstateManager::SnapshotBlockhash() return semantics (James O'Beirne)
7a6c46b37edb8bfa0085d202aa7e9427d5e4fceb chainparams: add allowed assumeutxo values (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

  ---

  This change proposes logic for activating UTXO snapshots, which is unused at the moment aside from an included unittest. There are a few moveonyish/refactoring commits to allow for halfway decent unittests.

  Basic structure is included for specifying and checking the assumeutxo hash values used to validate activated snapshots. Initially I had specified a few height/hash pairs for mainnet in this change, but because of the security-critical nature of those parameters, I figured it was better to leave their inclusion to a future PR that includes only that change - my intent being that reviewers will be more likely to verify those parameters firsthand in a dedicated PR.

  Aside from that and the snapshot activation logic, there are a few related changes:

  - ~~allow caching the `nChainTx` value in the CCoinsViewDB; this is set during snapshot activation. Because we don't necessarily have access to the full chain at the time of snapshot load, this value is communicated through the snapshot metadata and must be cached within the chainstate to survive restarts.~~
  - break out `CreateUTXOSnapshot()` from dumptxoutset. This is essentially a move-only change to allow the reuse of snapshot creation logic from within unittests.
  - ...and a few other misc. changes that are solely related to unittests.

  The move-onlyish commit is most easily reviewed with `--color-moved=zebra`.

ACKs for top commit:
  fjahr:
    Code review ACK 1afc0e4aa1b910991d4f8a77d74e2197f370987c
  laanwj:
    Code review ACK 1afc0e4aa1b910991d4f8a77d74e2197f370987c

Tree-SHA512: a4e4f0698f00a53ec298b5e8b7ef1c9fdf0185f95139d1b1f63cfdf6cbbd6d17b8c6e51bbf1de2e5f1a946bf49f8466232698ef55acce5a012c80b067da366ea

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