Replay MPP claims via background events using new CM metadata

Replay MPP claims via background events using new CM metadata

When we claim an MPP payment, then crash before persisting all the
relevant `ChannelMonitor`s, we rely on the payment data being
available in the `ChannelManager` on restart to re-claim any parts
that haven't yet been claimed. This is fine as long as the
`ChannelManager` was persisted before the `PaymentClaimable` event
was processed, which is generally the case in our
`lightning-background-processor`, but may not be in other cases or
in a somewhat rare race.

In order to fix this, we need to track where all the MPP parts of
a payment are in the `ChannelMonitor`, allowing us to re-claim any
missing pieces without reference to any `ChannelManager` data.

Further, in order to properly generate a `PaymentClaimed` event
against the re-started claim, we have to store various payment
metadata with the HTLC list as well.

Here we finally implement claiming using the new MPP part list and
metadata stored in `ChannelMonitor`s. In doing so, we use much more
of the existing HTLC-claiming pipeline in `ChannelManager`,
utilizing the on-startup background events flow as well as properly
re-applying the RAA-blockers to ensure preimages cannot be lost.