Log higher priority messages to stderr

We forward logs from docker containers to hosts journald in deployment. Docker apparently logs only stderr messages as critical. However, spdlog logs everything to stdout.

Probably we have to create two sinks for the worker loggers.

[1] https://github.com/docker/docker-ce/blob/96d50fd61df5a62a26609523619f5a5386b7d6db/components/engine/daemon/logger/journald/journald.go#L122