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 [1] 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).