This is a first step away from sync-ids. We now check if replica and
primary are identical using sequence numbers when determining where to
allocate a replica shard.

If an index is no longer indexed into, issuing a regular flush will now
be enough to ensure a no-op recovery is done.

This has the nice side-effect of ensuring that closed indices and frozen
indices choose existing shard copies with identical data over
file-overlap comparison, increasing the chance that we end up doing a
no-op recovery (only no-op and file-based recovery is supported by
closed indices).

Relates elastic#41400 and elastic#33888

Supersedes elastic#41784

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Hopefully this makes test succeed in CI too.

Now lock during cleanup files to protect snapshotRecoveryMetadata from
seeing half copied data. snapshotRecoveryMetadata now handles peer
recovery and existing store recovery specifically, returning empty
snapshot in other recovery types (local shards, restore snapshot).

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