UDP transport layer for rnet-p2p

Wrote a UDP transport layer for rnet-p2p – connection management, handshake acknowlegements, and liveliness checks for dead peers (since no real time sockets in UDP).

Was fun.

PR: https://github.com/lla-dane/rnet/pull/8

PS: rnet-p2p now moved here: https://github.com/rnet-stack/rnet-p2p