faked "xdcr mutations left" stat

So that it doesn't get negative

Change-Id: I44d478a96116aebbf0b3d46c3cff7a6935a996fa
Reviewed-on: http://review.couchbase.org/34999
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>