introduce DebugLogger interface

[cenkalti]

Ref #61

@nictuku This is the smallest interface that I can think of: https://github.com/cenkalti/dht/blob/29206eba2e98f2d7fab5affe38d5655a78b45db7/logging.go#L3-L7

By implementing this interface people can still use dht with glog or any logger they want.

I have named it as Logger2 because there is already an interface named as Logger: https://github.com/cenkalti/dht/blob/29206eba2e98f2d7fab5affe38d5655a78b45db7/dht.go#L223-L227

Of course this name is unacceptable :)

In fact, existing Logger interface does not represent a logger actually. I suggest renaming it as Hooks or something else and make Logger2 -> Logger. However, this is not possible without breaking backwards compatibility. I think it is best to tag the current commit as v1 and include this change in v2.

[nictuku]

Thanks for the PR. Could you document Logger and Logger2 in the Config struct? Feel free to vocally complain about Logger and say it should be renamed to Hooks 😅.

[nictuku]

For record, this change removes some granularity about log message verbosity. But that's OK since in the worst case scenario (e.g: when debugging) one can just log everything and filter things later.

[cenkalti]

@nictuku I think Logger2 is really a bad name :) Can I rename it as MessageLogger?

[nictuku]

Perhaps DebugLogger ?

[cenkalti]

@nictuku done.

[nictuku]

@cenkalti the change looks good. I have no further comments.

The CI is broken, sadly, so I'll test it out tomorrow or so, then I'll merge.

Thank you again for this Cenk!