Change win32 child-death tracking code to use a threadpool to wait for

Change win32 child-death tracking code to use a threadpool to wait for
childprocess deaths instead of using one thread per child. This drastastically
reduces the address space usage and should allow for more backends running.

Also change the win32_waitpid functionality to use an IO Completion Port for
queueing child death notices instead of using a fixed-size array.