MB-8937: fix internal setting of "xdcrOptimisticReplicationThreshold"

For parameter "xdcrOptimisticReplicationThreshold", the default value
is 256, and it should show up in internal settings.

Change-Id: Ib379a6dbbe1dec890b9ab9b7feff22ca73685c43
Reviewed-on: http://review.couchbase.org/28480
Tested-by: Junyi Xie <junyi.couchbase@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>