Merge #18637: coins: allow cache resize after init

f19fdd47a6371dcbe0760ef6f3c3c5adb31b1bb4 test: add test for CChainState::ResizeCoinsCaches() (James O'Beirne)
8ac3ef46999ed676ca3775f7b2f461d92f09a542 add ChainstateManager::MaybeRebalanceCaches() (James O'Beirne)
f36aaa6392fdbdac6891d92202d3efeff98754f4 Add CChainState::ResizeCoinsCaches (James O'Beirne)
b223111da2e0e9ceccef75df8a20252b0094b7bc txdb: add CCoinsViewDB::ChangeCacheSize (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 the assumeutxo implementation draft (#15056), once a UTXO snapshot is loaded, a new chainstate object is created after initialization. This means that we have to reclaim some of the cache that we've allocated to the original chainstate (per `dbcache=`) to repurpose for the snapshot chainstate.

  Furthermore, it makes sense to have different cache allocations depending on which chainstate is more active. While the snapshot chainstate is working to get to the network tip (and the background validation chainstate is idle), it makes sense that the snapshot chainstate should have the majority of cache allocation. And contrariwise once the snapshot has reached network tip, most of the cache should be given to the background validation chainstate.

  This set of changes (detailed in the commit messages) allows us to dynamically resize the various coins caches. None of the functionality introduced here is used at the moment, but will be in the next AU PR (which introduces `ActivateSnapshot`).

  `ChainstateManager::MaybeRebalanceCaches()` defines the (somewhat normative) cache allocations between the snapshot and background validation chainstates. I'd be interested in feedback if anyone has thoughts on the proportions I've set there.

ACKs for top commit:
  ajtowns:
    weak utACK f19fdd47a6371dcbe0760ef6f3c3c5adb31b1bb4 -- didn't find any major problems, but not super confident that I didn't miss anything
  fjahr:
    Code review ACK f19fdd4
  ryanofsky:
    Code review ACK f19fdd47a6371dcbe0760ef6f3c3c5adb31b1bb4. Only change since last review is constructor cleanup (no change in behavior). I think the suggestions here from ajtowns and others are good, but shouldn't delay merging the PR (and hold up assumeutxo)

Tree-SHA512: fffb7847fb6993dd4a1a41cf11179b211b0b20b7eb5f7cf6266442136bfe9d43b830bbefcafd475bfd4af273f5573500594aa41fff03e0ed5c2a1e8562ff9269

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