Release a hashtable partition lock before UPR/TAP notification.

This change removes the unnecessary locking overhead of hashtable
partition lock while notifying UPR/TAP producer of an arrival of
a mutation.

Change-Id: I186dbb6a94438290557bf6456b1c5ccb8a2e4484
Reviewed-on: http://review.couchbase.org/39239
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>