ReplicatedBackend: take read locks for clone sources during recovery

ReplicatedBackend: take read locks for clone sources during recovery

Otherwise, we run the risk of a clone source which hasn't actually
come into existence yet being used if we grab a clone which *just*
got added the the ssc, but has not yet actually had time to be
created (can't rely on message ordering here since recovery messages
don't necessarily order with client IO!).

Fixes: http://tracker.ceph.com/issues/17831
Signed-off-by: Samuel Just <sjust@redhat.com>