Handle duplicate payment claims during initialization

Handle duplicate payment claims during initialization

In the next commit we'll start using (much of) the normal HTLC
claim pipeline to replay payment claims on startup. In order to do
so, however, we have to properly handle cases where we get a
`DuplicateClaim` back from the channel for an inbound-payment HTLC.

Here we do so, handling the `MonitorUpdateCompletionAction` and
allowing an already-completed RAA blocker.