Protocol for multiple in-flight updates.

They concluded that it is more like a request "staging" ID and that two IDs sent in the commitment message allow for simultaneous action on accept/reject/etc. It's important to note that all messages sent must be in order, and if the replyer skips the requester's Add Requests when replying, the skipped are assumed to be request rejections (or an outright channel closeout) since it should never happen. This is to enforce accept/reject order as we need to know which modifications are included in the signature/transaction and not have that change after-the-fact.