CBQE-0: improve memcached kill

Change-Id: I16b2f124407c15baab389ae697cc440c5492077d
Reviewed-on: http://review.couchbase.org/37411
Reviewed-by: Thuan Nguyen <soccon@gmail.com>
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Parag Agarwal <agarwal.parag@gmail.com>
Tested-by: Parag Agarwal <agarwal.parag@gmail.com>