[gen_server2] Allow aborting jobs for a particular queue.
[gen_server2] Allow aborting jobs for a particular queue.

Two functions are provided:

 - abort_queue/3 aborts all jobs on the queue and passes its second
   argument as a result to all handle_result-s. That lets the job to
   respond to outside processes waiting for it (if any)

 - abort_queue/1 simply aborts all jobs on the queue without any extra
   cleanup

Change-Id: Icc084a82a431b42126f330ce209d39addf33ebba
Reviewed-on: http://review.couchbase.org/88116
Tested-by: Aliaksey Artamonau <ce14b1c16e0fd654f926118eb8addbec5aff4ffa@couchbase.com>
Well-Formed: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Artem Stemkovski <15a0730a27c69f62ab86f2b6c639fca202166c02@couchbase.com>

membase/ns_server,membase/ns_server,membase/ns_server,membase/ns_server,membase/ns_server,membase/ns_server