Refreshed BIP324

In a discussion on the Bitcoin-dev mailing list, it was suggested that short 1-byte message type IDs could be used with the top bit reserved for indicating when a 2-byte ID is required. This approach is similar to how integers are stored in protobuf. Changes from this discussion have now been incorporated into a pull request for the bips repository on GitHub. The discussion also touched upon the negotiation process for mapping tables and the possibility of simplifying implementations by allowing nodes to choose their own short ID mappings from day one.