Store info about claimed payments, incl HTLCs in `ChannelMonitor`s

Store info about claimed payments, incl HTLCs in `ChannelMonitor`s

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 store the required MPP parts and metadata in
`ChannelMonitor`s and make them available to `ChannelManager` on
load.