Fixes potential 'too many open files error' when running all servers.

Related to https://groups.google.com/forum/#!topic/edx-code/p8N8WteHRHI

@singingwolfboy I think the problem is that there were two watchdog threads starting if you were to run `paver run_all_servers`. I couldn't reproduce the error, but it seems like that's where the problem lies.