MB-8351: Expose optimistic XDCR stats on UI

This commit will add a new XDCR stat in outbound XDCR
stat section showing the number of docs that have been
replicated optimistically to remote cluster. It is an
aggregated stat from stat reported by each vb replicator.

Change-Id: I194ddf1cd5c823f599372a206372fce218f8a691
Reviewed-on: http://review.couchbase.org/28106
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Junyi Xie <junyi.couchbase@gmail.com>