p2p: Define a specification

[thanethomson]

This follows from #9089.

We want to ensure that our P2P layer implementation is correct. Given that all we have at present (v0.37, v0.34 and earlier versions) is an implementation, without a specification (i.e. a definition of how the P2P layer should work), it makes it hard to understand and test.

To this end, we believe we need to develop (i.e. invent) a specification for the P2P layer, using whatever relevant learnings we've picked up from our prior work in documenting how the v0.34 P2P layer works. This can also be informed by ADR-061 and ADR-062. This need not apply to the entire P2P layer - only those parts that can possibly be specified.

Follow-up work to this will include refactoring of the P2P layer to conform to the specification we develop here.