Fetch stats asynchronously so slow services don't stop the dashboard from being displayed

[ansell]

If a service is so slow that its results never make it into the cache, the dashboard is unresponsive or very slow, as happened today when the digivol stats were not returning results.

Calls to the very expensive stats web services should be performed asynchronously so the dashboard is still responsive.

It would also be useful to verify that only a single call to each web service is active at each time, so as not to create a DoS on each server by asking for the very expensive stats calls multiple times.