UDP support

[cannium]

First step for libp2p/go-libp2p#433,
also change package name to reusetransport;
no interface should be (accidentally) changed.

Btw, the tests are of very high quality and helped me a lot.

[Stebalien]

First of all, I apologize for not getting back to you sooner on that issue. This looks like an excellent patch. Unfortunately, I'm not sure UDP reuseport is actually useful (for us, at least)

We need reuseport for TCP connections to:

1. Open outbound connections on a port we're already listening on.
2. Open multiple outbound connections from the same port.

However, UDP doesn't have connections. We can reuse the same port (and even the same underlying OS socket) by calling pc := net.ListenPacket("udp", ":NNNN"). From here, we can send packets to any ip/port with pc.WriteTo(buf, dest) and read incoming packets from any ip/port with pc.ReadFrom(buf).

The issue with QUIC is libp2p/go-libp2p-quic-transport#8. Basically, we just need to extract the port picking logic from this package and then reuse it in go-libp2p-quic-transport when trying to pick a connection to dial with.

[cannium]

I make a pull request to QUIC transport: libp2p/go-libp2p-quic-transport#34, please review @Stebalien.
Sorry for the long response time, I took a vacation recently, so...
Close this pull request for now.

[Stebalien]

No need to apologize for response times, ever.