This commit morphs the prior CancelHTLC into the new UpdateFailHTLC
message and also gets rid of the obsolete HLTCAddReject message while
we’re at it.
The primary change from the CancelHTLC message to the UpdateFailHTLC
message is that the CancelReason is now simply called Reason and that
it’s now an opaque encrypted set of bytes. With this update the failure
messages are now more flexible (they can even carry new
ChannelUpdate’s) and also don’t reveal the exact cause of failure to
intermediate nodes.
