Speed up NAT status assertion time

[raulk]

Currently it may take in order of minutes to determine our NAT status. This can make the application sit idle for that much time before it can do anything useful, if it needs to make itself reachable.

Some ideas to speed up our NAT assertion time:

• client-side: parallelise dialback requests.
• client-side: AutoNAT public status: if we receive a positive answer from one AutoNAT server, roll over only the successful address to remaining AutoNAT servers; kinda like a cross-check vs. full check.
• client-side/server-side: start with low timeouts, and increase as necessary.

Relates to libp2p/go-libp2p#518.