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.

[1] https://github.com/docker/docker-ce/blob/96d50fd61df5a62a26609523619f5a5386b7d6db/components/engine/daemon/logger/journald/journald.go#L122
9 jobs from dev-tp in 9 minutes and 4 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Prepare
passed #198592
prepare:activemqcpp

00:00:27

 
  Build
passed #198594
build:webui

00:03:14

passed #198593
build:worker

00:00:44

 
  Test
passed #198597
test:webui

00:01:00

passed #198595
test:worker

00:00:23

passed #198596
test:worker:clang-format

00:00:15

 
  Docker Build
passed #198599
docker-build:client

00:04:20

passed #198598
docker-build:server

00:02:31

passed #198600
docker-build:worker

00:01:46