Spawning server pegs CPU at 100%

The Shell app (HUE-141) introduced the spawning server. Ever since, there're lot of report of the server pegging a CPU at 100%. To reproduce using curl:

## Get a cookie
$ curl -d 'username=foo&password=bar' -c /tmp/cookie.jar -i http://localhost:8000/accounts/login/

## Stress the app
$ while true; do curl -b /tmp/cookie.jar http://localhost:8000/filebrowser/view/tmp >& /dev/null; sleep 0; echo -n . ; done

Then start a Shell in the web ui and use it a bit. Stop the curl and the spawning server is still pegged at 100%.