Merge bitcoin/bitcoin#24138: index: Commit MuHash and best block together for coinstatsindex

691d45fdc83ec14f87a400f548553168ac70263f Add coinstatsindex_unclean_shutdown test (Ryan Ofsky)
eb6cc05da32c5bde122725a0bc907d3767a791cd index: Commit DB_MUHASH and DB_BEST_BLOCK to disk together (Martin Zumsande)

Pull request description:

  Fixes #24076

  Coinstatsindex currently writes the MuHash (`DB_MUHASH`) to disk in `CoinStatsIndex::WriteBlock()` and `CoinStatsIndex::ReverseBlock()`, but the best synced block is written in `BaseIndex::Commit()`. These are called at different points in time, both during the ThreadSync phase, and also after the initial sync is finished and validation callbacks (`BlockConnected()` vs `ChainStateFlushed()`) perform the syncing.

  As a result, the index DB is temporarily in an inconsistent state, and if bitcoind is terminated uncleanly (so that there is no time to call `Commit()` by receiving an interrupt or by flushing the chainstate) this leads to problems:
  On the next startup, `Init()` will read the best block and a MuHash that corresponds to a different (higher) block. Indexing will  be picked up at the the best block processing some blocks again, but since MuHash is a rolling hash, it will process some utxos twice and the muhashes for all future blocks will be wrong, as was observed in #24076.

  Fix this by always committing `DB_MUHASH` together with `DB_BEST_BLOCK`.

  Note that the block data for the index is still written at different times, but this does not corrupt the index - at worst, these entries will be processed another time and overwritten after an unclean shutdown and restart.

ACKs for top commit:
  ryanofsky:
    Code review ACK 691d45fdc83ec14f87a400f548553168ac70263f. Only change since last review is adding test
  fjahr:
    ACK 691d45fdc83ec14f87a400f548553168ac70263f

Tree-SHA512: e1c3b5f06fa4baacd1b070abb0f8111fe2ea4a001ca8b8bf892e96597cf8b5d5ea10fa8fb837cfbf46648f052c742d912add4ce26d4406294fc5fc20809a0e1b

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