Refactor runtime device tracker

Made several changes (improvements) to the runtime device tracker. One important change is that the Reset method is compiled into the vtkm_cont library. This way you can use RuntimeDeviceTracker with other functions compiled in the library without having to compile for devices (such as CUDA) yourself.

Another important change is the addition of a global runtime device tracker. Now we can generally make functions that try several devices and sync up the monitoring of which devices are working on the system.