MB-9953 Replace the old queued_item instance with the new one.

When a new mutation arrives into a checkpoint, we should replace
the old queued_item instance with a new instance for the same
key. Otherwise, some fields won't be correctly updated (e.g.,
rev_seqno is not updated correctly).

Change-Id: I9d7c67be675c2297c84e253c47941149dd76d406