Because concurrent sync requests from a primary to its replicas could be
in flight, it can be the case that an older retention leases collection
arrives and is processed on the replica after a newer retention leases
collection has arrived and been processed. Without a defense, in this
case the replica would overwrite the newer retention leases with the
older retention leases. This commit addresses this issue by introducing
a versioning scheme to retention leases. This versioning scheme is used
to resolve out-of-order processing on the replica. We persist this
version into Lucene and restore it on recovery. The encoding of
retention leases is starting to get a little ugly. We can consider
addressing this in a follow-up.
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