Synchronize UPRProducer::handleResponse API through the lock.

Change-Id: I782d8e0c126a23174d50eb89c628f91d791dbe39
Reviewed-on: http://review.couchbase.org/38995
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>