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.
'init' message is the first message reveals the features supported or required
by this node. Nodes wait for receipt of the other's features to simplify error
diagnosis where features are incompatible. This message will help negotioate the
the features which are supported by both sides.
