crash while dragging or previewing

This crash can happen at any time but it's primarily while dragging or previewing. The crash reports are all over the place -- but usually in a non-crashing thread a timer is being updated or freed.

I suspect these mach timers have some non-threadsafe component -- which is pretty frustrating since measuring multi-threaded performance is really what i was using them for.

For the time being I've corrected this by totally disabling them except when developers enable them.