Improvements to upstart job definition file

- Don't start at startup, since network interfaces may not be ready.
  Instead start when the first non-loopback network interface is up.
- Use Upstart's setgid / setuid directives to run as _statsd user.
- Omit implicit 'stop on shutdown'.
- Add a conventional & friendly comment header.
- Respawn StatsD on abnormal termination, but give up after 10 tries.
- Do not redirect StatsD's output to /var/log/statsd/statsd.log. Instead
  simply allow Upstart to capture output and save it to
  /var/log/upstart/statsd.log.