worker: Add graceful shutdown

[kpelelis]

The graceful shutdown mechanism is only available for clustered installations (e.g. devMode = true). In our production stack, we are currently using multiple (HAProxy managed) single threaded instances of hypernova (e.g. devMode = true). This instances are actually systemd units so our init script is something along the lines of

# Start worker(s)
start_nova_workers() {
    for worker in $(nova_workers); do
      echo "Start ${worker}"
      systemctl start ${worker}
    done
  fi
}

Shutdown and restart is similar.

When we want to updated these instances, we do a rolling restart with HAProxy, which kills signals to each worker, in a round-robin manner, and re spawns it. There is a race condition in this shutdown process with open connections (as there is in the clustered version of hypernova). This patch introduces the same functionality with the single core installation.

[kpelelis]

Thanks a lot for merging this :) Is there a release schedule?