Refreshed BIP324

In a Bitcoin-dev mailing list, Pieter Wuille proposed using the BIP process to introduce a fixed initial table and allowing future BIPs to introduce new mapping entries for it. Murch suggested using the first bit to signal a 2-byte message ID, which would allow the expression of 128 IDs with 1 byte and a total of 2^15 IDs across 2 bytes if needed. He also expressed that collisions could be announced to the mailing list as part of the corresponding BIP and any overlooked collision should become apparent at implementation time. To further mitigate risks of collisions, less prevalent message types could utilize a 2-byte ID.