Try to throttle using a critical section.

The first implementation of service restart throttling used a condition
variable in a critical section to sleep for the required amount of time.
The implementation was changed to use a waitable timer because Windows
2000 does not support SleepConditionVariableCS() or
WakeConditionVariable().

Since we are now using LoadLibrary() and GetProcAddress() to use newer
functions dynamically without having to build OS-specific binaries, we
can now use a critical section where it's supported and fall back to a
waitable timer when running on Windows 2000.