SERVER-9580 Fixed problem with sleepmillis() on Windows due to default timer resolution being typically higher than 15ms

All sleepmillis() calls with values lesser than timer resolution will sleep AT LEAST the timer resolution value, making Mongo be really slow on certain use cases. Like when implementing consumer/producer patterns using capped collections to stream objects back and forth
@benety , sorry, I don't understand your comment. Are you saying this declaration shall be available on some msdn include file?
I tried to avoid full inclusion of <Windows.h> header file. The approach was minimally invasive to the code just declaring the extern.