all: direct connections

[dylan-tailscale]

Implement direct connections as a transport, along with the required Disco and STUN implementations. Must be able to handle multiple direct connections per peer, for multiple peers.

[j-shawts]

High level breakdown:

• Implementing discovery protocol (may be partially done)
• Implementing NAT traversal state machine (possible DERP work?)
• Implementing port mapping support
• Adding STUN latency checking
• "glue work"