Unify libp2p log format

[jannikluhn]

We have two main sources of logs using different libraries:

• our own rolling shutter logs using zerolog
• libp2p logs using go-log based on zap

We can already configure the log levels using a single environment variable ROLLING_SHUTTER_LOGLEVEL which is read in setupLogging. However, they use two different output formats which we should unify.

A crude way to do this is to set the go-log format to JSON and define a zap sink that decodes the JSON messages and relogs them with zerolog.