Initial pass at moving the WMISampler onto its own thread.  The thread
polls wmi at the 15 second interval.  The various checks then take the
last good reading.  This has two effects:

-  The checks are always non-blocking

-  The data they read could be older than expected (if the WMI request
   takes a long time)

   Added stop() to Check() class, and call check from collector to shut
   down

   WMISampler thread.

   Add WMICheck::stop(), and call WMISampler stop for each sampler