Don't schedule notifications for backfill, send it immeidiately instead

Since backfills run in a ep-engine thread we can notify the memcached
layer immediately without having to schedule a seperate task.

Change-Id: I75675028e8576c18a832d55b52773d39b8b7419a
Reviewed-on: http://review.couchbase.org/35127
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>