sockfilt.c: Replace 100ms sleep with thread throttle

Improves performance of test cases 574 and 575 by 50%.

A value of zero causes the thread to relinquish the remainder
of its time slice to any other thread of equal priority that is
ready to run. If there are no other threads of equal priority
ready to run, the function returns immediately, and the thread
continues execution.

http://msdn.microsoft.com/library/windows/desktop/ms686307.aspx