Refactor payment-claim logic to ensure MPP-claim atomicity

Refactor payment-claim logic to ensure MPP-claim atomicity

Previously if we claimed an MPP where a previous-hop channel was
closed while we were waitng for the user to provide us the preimage
we'd simply skip claiming that HTLC without letting the user know.

This refactors the claim logic to first check that all the channels
are still available (which is actually all we need - we really
mostly care about updating the channel monitors, not the channels
themselves) and then claim the HTLCs in the same lock, ensuring
atomicity.