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
MB-11592: Create snapshots when getting tiems from the checkpoint queues

Change-Id: I8a262eb7856c97e11e95570fbdaef82f3f79054a