[Backport] Fix deadlock in checkpoint persistence command

We need to release the hpChkMutex before notifying memcached in
order to prevent a deadlock. The deadlock occurs when the flusher
trys to notify memcached of checkpoint persistence at the same
time a memcached worker thread is trying to add a new checkpoint
persistence request.

Change-Id: Ida313f5b39ef0e063dee9882410cd0a19ce55292
Reviewed-on: http://review.couchbase.org/41363
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>
Reviewed-on: http://review.couchbase.org/42407
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>