swarm: introduce address tiers to prioritize dials

[marten-seemann]

Not all addresses are created equal, and we should be smarter when dialing:

• We should prefer IPv6 over IPv4, implementing an Happy-Eyeballs-like mechanism as described in RFC 8305.
• When given an IP multiaddr and a DNS multiaddr to dial (for any given transport), we should start dialing the IP address immediately, and start DNS resolution concurrently.
• We should prefer QUIC over TCP. QUIC handshakes are fast (1 RTT), so giving QUIC a head start of a few hundred ms will allow us to not start the TCP handshake in the vast majority of cases.
• We need to be able to learn from failed dials. Sometimes IPv6 is blackholed, or UDP. This blackholing will apply to address categories (it might work on the local network, but fail on the public internet), so the algorithm needs to be able to classify addresses.

Things to consider: it might be worth considering changing our transport interface, such that Dial is async and can be called with multiple addresses.

[marten-seemann]

Left to do to complete this effort:

• smart dialing is implemented
• detect UDP blackholes, and disable QUIC if detected
• detect IPv6 blackholes, and disable IPv6 if detected, otherwise prioritize dials on IPv6 (RFC 8305)
• metrics for blackhole detection
• enable smart dialing by default

[sukunrt]

@marten-seemann we don't need to explicitly disable QUIC or IPv6, right? If they're black holed we will not dial those addresses and we won't establish any incoming connections either.

[marten-seemann]

Closing this issue, since @sukunrt has successfully chewed through all of the above points. We still need to publish a blog post. I created libp2p/blog#92 to track that.