MB-12706: clear temporary item id on an add operation

When an add operation is performed on a temporary item, the id needs
to be cleared so that it isn't temporary item anymore.

Change-Id: Iefde4c74ec5c836cdd67404331358f3bd062ee2e
Reviewed-on: http://review.couchbase.org/43383
Tested-by: Sriram Ganesan <sriram@couchbase.com>
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>