BIP 324: Clarify equivalence between 1 and 13 byte message types

BIP 324 introduced single-byte short identifiers for messages used multiple times in a connection, relating them to existing v1 messages, however it left it ambiguous as to what should happen if a 13-byte message_type was received. Clarify that the single byte message types are simply aliases for the longer equivalents, and that nodes should behave identically whether a message uses a one byte message_type or the equivalent 13-byte message type.

Also lowercases the message types (matching the on-the-wire encoding, rather than the Bitcoin Core constants), explicitly defines the payload's message_length, and adds a Version header and Changelog section.