index: Improve BaseIndex::BlockUntilSyncedToCurrentChain reliability
index: Improve BaseIndex::BlockUntilSyncedToCurrentChain reliability

Since commit f08c9fb0c6a799e3cb75ca5f763a746471625beb from PR
https://github.com/bitcoin/bitcoin/pull/21726, index
`BlockUntilSyncedToCurrentChain` behavior has been less reliable, and there has
also been a race condition in the `coinstatsindex_initial_sync` unit test.

It seems better for `BlockUntilSyncedToCurrentChain` to actually wait for the
last connected block to be fully processed, than to be able to return before
prune locks are set, so this switches the order of `m_best_block_index =
block;` and `UpdatePruneLock` statements in `SetBestBlockIndex` to make it more
reliable.

Also since commit f08c9fb0c6a799e3cb75ca5f763a746471625beb, there has been a
race condition in the `coinstatsindex_initial_sync` test. Before that commit,
the atomic index best block pointer `m_best_block_index` was updated as the
last step of `BaseIndex::BlockConnected`, so `BlockUntilSyncedToCurrentChain`
could safely be used in tests to wait for the last `BlockConnected`
notification to be finished before stopping and destroying the index. But
after that commit, calling `BlockUntilSyncedToCurrentChain` is no longer
sufficient, and there is a race between the test shutdown code which destroys
the index object and the new code introduced in that commit calling
`AllowPrune()` and `GetName()` on the index object. Reproducibility
instructions for this are in
https://github.com/bitcoin/bitcoin/issues/25365#issuecomment-1259744133

This commit fixes the `coinstatsindex_initial_sync` race condition, even though
it will require an additional change to silence TSAN false positives,
https://github.com/bitcoin/bitcoin/pull/26188, after it is fixed. So this
partially addresses but does not resolve the bug reporting TSAN errors
https://github.com/bitcoin/bitcoin/issues/25365.

There is no known race condition outside of test code currently, because the
bitcoind `Shutdown` function calls `FlushBackgroundCallbacks` not
`BlockUntilSyncedToCurrentChain` to safely shut down.

Co-authored-by: Vasil Dimov <vd@FreeBSD.org>
Co-authored-by: MacroFake <falke.marco@gmail.com>

Github-Pull: #26215
Rebased-From: 8891949bdcb25093d3a6703ae8228c3c3687d3a4

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