When restoring an htlc fulfill update from disk, the payment hash wasn't
restored previously.

…te updates

This test asserts that remote updates that are locked-in on the local
commitment, but haven't been signed for on the remote commitment, are
properly restored after a restart.

This method is only used to update the local commitment transaction.
Updated comment accordingly.

Extract functionality to methods as a preparation for serializing remote
log updates.

Extract method in preparation for restoring dangling remote updates. We
need to get rid of the early return.

This commit updates the channel state machine to
persistently store remote updates that we have received a
signature for, but that we haven't yet included in a commit
signature of our own.

Previously those updates were only stored in memory and
dropped across restarts. This lead to the production of
an invalid signature and channel force closure. The remote
party expects us to include those updates.