WIP: Peer Discovery DEP

[pfrazee]

This one is almost ready to go. Needs review and a few TODO items which should be easy to resolve.

TODOs

• [ ] add a section about hole punching
• [x] add a section about the discovery key
• [ ] fix possible amplification attack?
• Some things Maf and I plan to change in the implementation in a debug session soon:
  • [ ] fix encoding to include all 32 bytes (not the 20 currently)
  • [ ] switch to blake2b in token gen
  • [ ] rethink how push works to make sure we notify (almost) everyone, or possibly switch to pull only
  • [ ] consider a new hole-punching system that replaces push announcements

[pfrazee]

Ok I covered most feedback (thanks all!). A few TODOs are written in the Unresolved questions. There's also some changes that we're going to implement in March, so I'm going to leave this DEP for now.