Merge bitcoin/bitcoin#25667: assumeutxo: snapshot initialization

bf95976061 doc: add note about snapshot chainstate init (James O'Beirne)
e4d7995286 test: add testcases for snapshot initialization (James O'Beirne)
cced4e7336 test: move-only-ish: factor out LoadVerifyActivateChainstate() (James O'Beirne)
51fc9241c0 test: allow on-disk coins and block tree dbs in tests (James O'Beirne)
3c361391b8 test: add reset_chainstate parameter for snapshot unittests (James O'Beirne)
00b357c215 validation: add ResetChainstates() (James O'Beirne)
3a29dfbfb2 move-only: test: make snapshot chainstate setup reusable (James O'Beirne)
8153bd9247 blockmanager: avoid undefined behavior during FlushBlockFile (James O'Beirne)
ad67ff377c validation: remove snapshot datadirs upon validation failure (James O'Beirne)
34d1590331 add utilities for deleting on-disk leveldb data (James O'Beirne)
252abd1e8b init: add utxo snapshot detection (James O'Beirne)
f9f1735f13 validation: rename snapshot chainstate dir (James O'Beirne)
d14bebf100 db: add StoragePath to CDBWrapper/CCoinsViewDB (James O'Beirne)

Pull request description:

  This is part of the [assumeutxo project](https://github.com/bitcoin/bitcoin/projects/11) (parent PR: https://github.com/bitcoin/bitcoin/pull/15606)

  ---

  Half of the replacement for #24232. The original PR grew larger than expected throughout the review process.

  This change adds the ability to initialize a snapshot-based chainstate during init if one is detected on disk. This is of course unused as of now (aside from in unittests) given that we haven't yet enabled actually loading snapshots.

  Don't be scared! There are some big move-only commits in here.

  Accompanying changes include:

  - moving the snapshot coinsdb directory from being called `chainstate_[base blockhash]` to `chainstate_snapshot`, since we only support one snapshot in use at a time. This simplifies some logic, but it necessitates writing that base blockhash out to a file within the coinsdb dir. See [discussion here](https://github.com/bitcoin/bitcoin/pull/24232#discussion_r832762880).
  - adding a simple fix in `FlushBlockFile()` that avoids a crash when attemping to flush to disk before `LoadBlockIndexDB()` is called, which happens when calling `MaybeRebalanceCaches()` during multiple chainstate init.
  - improving the unittest to allow testing with on-disk chainstates - necessary to test a simulated restart and re-initialization.

ACKs for top commit:
  naumenkogs:
    utACK bf95976061
  ariard:
    Code Review ACK bf9597606
  ryanofsky:
    Code review ACK bf95976061. Changes since last review: rebasing, switching from CAutoFile to AutoFile, adding comments, switching from BOOST_CHECK to Assert in test util, using chainman.GetMutex() in tests, destroying one ChainstateManager before creating a new one in tests
  fjahr:
    utACK bf95976061
  aureleoules:
    ACK bf95976061

Tree-SHA512: 15ae75caf19f8d12a12d2647c52897904d27b265a7af6b4ae7b858592eeadb8f9da6c2394b6baebec90adc28742c053e3eb506119577dae7c1e722ebb3b7bcc0

doc/design/assumeutxo.md

@@ -76,8 +76,9 @@ original chainstate remains in use as active.
 
 Once the snapshot chainstate is loaded and validated, it is promoted to active
 chainstate and a sync to tip begins. A new chainstate directory is created in the
-datadir for the snapshot chainstate called
-`chainstate_[SHA256 blockhash of snapshot base block]`.
+datadir for the snapshot chainstate called `chainstate_snapshot`. When this directory
+is present in the datadir, the snapshot chainstate will be detected and loaded as
+active on node startup (via `DetectSnapshotChainstate()`).
 
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