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.
Change the name of fields of messages which are belong to the discovery
subsystem in a such way so they were the same with the names that are
defined in the specification.
In this commit announcement signature message has been added which is
needed when peers want to announce their channel to the rest of the
network. This message acts as half proof carrier, nodes exchanges
their half proofs with each other and after that they are able to
construct the full proof.
