Implement supervisor system

Because serverengine uses signal based start/stop mechanism.

The mechanism is difficult to implement graceful stop/restart. Because
we can't get stopped notification from worker processes. We need to
create a pipe to get notifications from worker processes by ourselves.

And serverengine uses the original logging mechanism. It is different
format with on droonga-engine.

The new supervisor system uses pipe to communicate with worker
processes. It can be got notifications from workers. We can implement
graceful stop/restart.

TODO:

  * Support auto-restart worker process when worker process is crashed.
    * Heartbeat should be implemented.
  * Support service ready notification for drntest.