MB-8038 Release hash table partition lock before notifyIOComplete

There can be a deadlock case where a memcached worker thread
tries to grab a hash table partition lock, while the dispatcher
thread already grabs the same parition lock and tries to notify
the IO complete event to the above worker thread.

Change-Id: I550f02f315d0741f300fdb09fe903fcabb1ef4b8
Reviewed-on: http://review.couchbase.org/25834
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>