Add BIP324: v2 P2P Transport Protocol

[dhruv]

Supersedes #1024

Link to rendered version for your convenience: https://github.com/dhruv/bips/blob/bip324/bip-0324.mediawiki

Open questions:

• The BIP text currently includes a table for 1-byte message type IDs. The process for updating this table atomically to assign new IDs when new message types are being proposed is likely going to be an annoyance. What are better ways to handle ongoing allocation?