Support graceful shutdown of Workhorse connections

This commit does two things:

1. Uses Go's graceful HTTP shutdown to close the listen port and wait for all HTTP handlers to finish. This will wait up to shutdown_timeout seconds, which is 0 by default, to preserve existing behavior.

2. Kicks out all long poll queries by closing down the shutdown channel, which is monitored by the Redis key watcher.

#325114 (closed)

Testing

1. Enable shutdown_timeout to be 60s.
2. Run workhorse with -apiCiLongPollingDuration 120s.
3. Run a register GitLab Runner. Verify it's sitting in long poll.
4. Fake a slow endpoint by adding sleep 30 to a controller (e.g. app/controllers/admin/dashboard_controller.rb)
5. Issue a SIGTERM or hit CTRL-C on Workhorse.
6. Observe the log message says shutdown initiated.
7. Observe that the /api/v4/jobs/request returns right away with a 204. Future requests fail.
8. Observe that Workhorse will hang out for 30 seconds while the endpoint loads.
9. Once the final request is loaded, Workhorse shuts down.