MB-8242: fix outbound XDCR lazy computed stats

Change-Id: If444c0369b832972b1f809f001e7126a577ac712
Reviewed-on: http://review.couchbase.org/26247
Tested-by: Junyi Xie <junyi.couchbase@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>