MB-11237: added dcp backoffs stat to ui

Change-Id: Ia67aded37940431fab527317e88190a4629472c6
Reviewed-on: http://review.couchbase.org/40125
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>