This commit modifies the Message interface to convert the Command
method to a MsgType method that uses a new set of message type for all
the defined messages. These new messages types nearly exactly match the
message types used within the current draft of the BOLT specifications.
This commit revues the Validate method from the Message interface as
the method is no longer used and is a relic from an older version of
the codebase.
This commit renames the prior CommitRevocation message to RevokeAndAck
in order to better align the set of wire messages implemented by the
`lnwire` message with those currently defined within the specification.
