CBD-837: add xdcr_optimistic_replication parameter

Change-Id: I1734ac053a6c8d4b22e96677292ce9fc8bf2d640
Reviewed-on: http://review.couchbase.org/24696
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Tested-by: Pavel Paulau <pavel.paulau@gmail.com>