MB-11592: Use a queue_item in a MutationResponse instead of and Item

This will allow us to pull queued_items out of the CheckpointManager
and into and ActiveStream without adding any memory overhead. This
also simplified the cleanup of a MutationResponse because we no
longer need to seperately delete the item contained inside this
class.

Change-Id: I0fc67ac66da6bd0c0ad0cd0369ee0a377d2b3bd9
Reviewed-on: http://review.couchbase.org/39283
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>