Better parallelize MPP claims #3986
Description
Currently if we have an MPP claim from multiple parts on the same channel, we'll walk the HTLCs and claim them totally in-order. This means we'll claim the first part from the first channel then have to wait until the peer responds with their RAA + CS until we can claim the next (or all remaining, hopefully) parts. It would be nice to claim them all in one go, which would reduce total claim latency and time spent waiting on monitor updates.