Ensure all HTLCs for a claimed payment are claimed on startup

Ensure all HTLCs for a claimed payment are claimed on startup

While the HTLC-claim process happens across all MPP parts under one
lock, this doesn't imply that they are claimed fully atomically on
disk. Ultimately, an application can crash after persisting one
`ChannelMonitorUpdate` out of multiple monitor updates needed for
the full claim.

Previously, this would leave us in a very bad state - because of
the all-channels-available check in `claim_funds` we'd refuse to
claim the payment again on restart (even though the
`PaymentReceived` event will be passed to the user again), and we'd
end up having partially claimed the payment!

The fix for the consistency part of this issue is pretty
straightforward - just check for this condition on startup and
complete the claim across all channels/`ChannelMonitor`s if we
detect it.

This still leaves us in a confused state from the perspective of
the user, however - we've actually claimed a payment but when they
call `claim_funds` we return `false` indicating it could not be
claimed.