Switch to env-logger or similar

[johanot]

Basically the same as: wharfix/wharfix#124

I'm fine with changing to env-logger in a single go, without having a deprecation period with feature-flags. But I'll let others decide on this :)

Switching to env-logger will probably allow a removal of the allow(dead-code) here: https://github.com/DBCDK/faythe/pull/95/files#diff-f49d609fea783d3a6a05bd81505337baf6575e8919cd505271cd501e91933809R244