MB-9741 Replace usage of QueuedItem with Item class.

This is the first phase of reducing memory used by keys/metadata of items.
This checkin now replace QueuedItem with Item used by Checkpoing Manager
and prepares for using Item class in StoredValue. QueuedItem is removed
from the code base.

Change-Id: I6afecac51410f209155cf357f6a276c20670b026
Reviewed-on: http://review.couchbase.org/31050
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>