MB-8937: fix internal setting of "xdcrOptimisticReplicationThreshold"
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 <cec9b1c9f34a8d92cba938f3f3364d5120580782@gmail.com>
Reviewed-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>

ceejatec/ns_server,daverigby/ns_server,vzasade/ns_server,membase/ns_server,t3rm1n4l/ns_server,t3rm1n4l/ns_server,vzasade/ns_server,mschoch/ns_server,ceejatec/ns_server,daverigby/ns_server,nimishzynga/ns_server,mschoch/ns_server,t3rm1n4l/ns_server,vzasade/ns_server,mschoch/ns_server,ceejatec/ns_server,mschoch/ns_server,nimishzynga/ns_server,membase/ns_server,membase/ns_server,nimishzynga/ns_server,membase/ns_server,nimishzynga/ns_server,daverigby/ns_server,ceejatec/ns_server,nimishzynga/ns_server,ceejatec/ns_server,daverigby/ns_server,daverigby/ns_server,nimishzynga/ns_server,mschoch/ns_server,vzasade/ns_server,t3rm1n4l/ns_server,membase/ns_server,ceejatec/ns_server,t3rm1n4l/ns_server,membase/ns_server,mschoch/ns_server