Issue 416 - Switch to Index change notifications

Prior to 1.4 ServiceControl made use of WaitForNonStaleIndexes when updating the counts for the ServicePulse dashboard.  These have been removed because they timed out when the bulk delete code was fixed.
Oren recommended we switch to the RavenDB Changes API and watch for index updates instead.

Relates to #416 
This returns an `IDisposable` that u should dispose of at `Stop`