worker: Allow logging to stderr
In deployment we use docker logs forwarded to journald. I'd really want to log directly into host's journal but that is a task for some other day.
Docker logs apparently  make the message's severity higher if it is parsed from stderr.
Worker will now log WARNING and higher to stderr if invoked with --log-stderr option. We modified spdlog's dist_sink to enable this functionality.
Closes #46 (closed).