bp:MB-8242: fix outbound XDCR lazy computed stats

Change-Id: I589627d669639e6083c5d7376a4ca704a87872bb
Orig-Change-Id: If444c0369b832972b1f809f001e7126a577ac712
[alk@tut.by: "backported" to 2.0.2]
Signed-off-by: Aliaksey Kandratsenka <alk@tut.by>
Reviewed-on: http://review.couchbase.org/26360
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>