don't turn off logging

[marten-seemann]

go-libp2p/p2p/discovery/mdns.go

Line 68 in cc70c4b

golog.SetOutput(ioutil.Discard)

turns off logging completely.
quic-go uses the go standard logger (but introduces a log level which can be set via an env variable, so quic-go is quiet by default). However, this line prevents quic-go from logging at all.

[marten-seemann]

The issue here is that mdns logs a lot. We definitely need a way to shut it up.

I see two practical ways to do so:

1. use go-log for mdns, and log everything at the lowest log level, i.e. debug. Maybe that's still too noisy though.
2. modify mdns such that it doesn't log any more. Shouldn't be too much work, it's just 11 log statements in total.

@whyrusleeping, wdyt?

[Stebalien]

Wow, yeah, we shouldn't do that. We've been trying to switch to a newer fork of our current MDNS library to fix some other issues (races) but... grandcat/zeroconf#38.

It may be worth it to just fork.