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 and modifies the CommitSignature message to more
closely match the CommitSig message defined within the current set of
draft specifications.
The major change within the new message is that we now longer
explicitly specify the update log index of the remote node that this
signature covers. This is due to the fact the revocation message now
also double as acknowledgements of the remote parties recevied
commitment update messages.
