Fix problems with prefetch and ConcurrentModuleTimer

If data was being prefetched for a module it could cause an assert
for ConcurrentModuleTimer to fail. The problem was the Service was
trying to 'stop' its clock when there was not a running module.
Merge pull request #3578 from Dr15Jones/fixPrefetchForConcurrentModuleTimer

Multithreading -- Fix problems with prefetch and ConcurrentModuleTimer