MB-8034: impl. some ui for xdcr_optimistic_replication_threshold

As well as primitive REST API

Change-Id: I20ef2916eb0db2df3a43966889ae67c212b634d1
Reviewed-on: http://review.couchbase.org/26003
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>