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 modifies the prior HTLCSettleRequest to more closely match
the UpdateFufillHTLC defined within the specification. The only
semantic change is the move from a slice of pre-images (for “multi-sig”
LN) to a single payment preimage.
