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

[ajtowns]

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.

[ajtowns]

cc @sipa @real-or-random @dhruv @jonasschnelli

I believe Bitcoin Core and btcd match this spec, but that rust-bitcoin does not, and will instead reject messages that don't use the assigned short id. (Likewise for floresta and the bip324 crate, AIUI)

The rationale is twofold:

• keep things simple and avoid creating two distinct namespaces for message types -- every message has a 12 byte name that's suitable for v1/v2, and some of those message have a short, one byte alias
• having short ids be an alias for the longer id makes dynamic allocation of short ids more straightforward: you just need to link the new short id with the 12-byte ASCII id; and if a short id is not allocated, you can reliably just use the 12-byte ASCII id.