Replaced MM timer with a high priority thread for audio compute, based on a patch...

Replaced MM timer with a high priority thread for audio compute, based on a patch by Dmitry Kostjuchenko. Reason: there is one global MM timer thread per process and doing audio compute in it was interfering with timer accuracy and also with running multiple DSound streams. The old MM timer based implementation is still available if you define PA_WIN_DS_USE_WMME_TIMER -- this may be removed in future.