Today if a shard is not fully allocated we maintain a retention lease for a
lost peer for up to 12 hours, retaining all operations that occur in that time
period so that we can recover this replica using an operations-based recovery
if it returns. However it is not always reasonable to perform an
operations-based recovery on such a replica: if the replica is a very long way
behind the rest of the replication group then it can be much quicker to perform
a file-based recovery instead.

This commit introduces a notion of "reasonable" recoveries. If an
operations-based recovery would involve copying only a small number of
operations, but the index is large, then an operations-based recovery is
reasonable; on the other hand if there are many operations to copy across and
the index itself is relatively small then it makes more sense to perform a
file-based recovery. We measure the size of the index by computing its number
of documents (including deleted documents) in all segments belonging to the
current safe commit, and compare this to the number of operations a lease is
retaining below the local checkpoint of the safe commit. We consider an
operations-based recovery to be reasonable iff it would involve replaying at
most 10% of the documents in the index.

The mechanism for this feature is to expire peer-recovery retention leases
early if they are retaining so much history that an operations-based recovery
using that lease would be unreasonable.

Relates #41536

back port of #45208