Track payments after they resolve until all HTLCs are finalized

Track payments after they resolve until all HTLCs are finalized

In the next commit, we will reload lost pending payments from
ChannelMonitors during restart. However, in order to avoid
re-adding pending payments which have already been fulfilled, we
must ensure that we do not fully remove pending payments until all
HTLCs for the payment have been fully removed from their
ChannelMonitors.

We do so here, introducing a new PendingOutboundPayment variant
called `Completed` which only tracks the set of pending HTLCs.