Lightning messages are identified by a 2-byte type when encoded on the
wire. Rather than expecting callers to know message types when sending
messages to peers, have each message implement a trait defining the
message type. Provide an interface for reading and writing messages
as well as a Message enum for matching the decoded message, including
unknown messages.
This adds a new fn to ChannelKeys which is called when we generte
a new remote commitment transaction for signing. While it may be
theoretically possible to unwind state updates by disconnecting and
reconnecting as well as making appropriate state machine changes,
the effort required to get it correct likely outweighs the UX cost
of "preflighting" the requests to hardwre wallets.
