discovery: increase default connection timeout

[lucab]

This commit increases default connection timeout from 5s to 20s,
in order to gracefully accomodate for retrials. This will help in
overcoming soft-failure cases, like skipping a single faulty
resolvers in resolv.conf.

This is still a quite aggressive timeout, well below the Linux
timeout of 127s for new TCP connections.

[lucab]

Coming from rkt/rkt#3009 (comment)

There is still some room for increase here, but I'm not sure about why the original value was picked so low (and #112 lacks a bit of context). For reference, nginx has 30s timeout just for DNS resolution while default Linux tcp_syn_retries result in 127s timeout for new connections.

[iaguis]

This looks reasonable to me (LRTM?)

[lucab]

@jonboulle may have some insights here.

[jonboulle]

LRTM