MB-11601 Fix to the memory leak in processing UPR messages.

If UPR_MUTATION fails to be sent or a message type is UPR_DELETION,
then we should free an Item instance after processing an
UPR_MUTATION or UPR_DELETION message.

Change-Id: I61849a1074f829b00d5191eb1417ee60059d4716
Reviewed-on: http://review.couchbase.org/39065
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>