mpm_winnt: Make the shutdown faster by avoiding unnecessary Sleep()'s

mpm_winnt: Make the shutdown faster by avoiding unnecessary Sleep()'s
when shutting down the worker threads.

Previously, the shutdown code was posting an amount of I/O completion
packets equal to the amount of the threads blocked on the I/O completion
port.  Then it would Sleep() until all these threads "acknowledge" the
completion packets by decrementing the global amount of blocked threads.

A better way would be to send the number of IOCP_SHUTDOWN completion
packets equal to the total amount of threads and immediately proceed to
the next step.  There is no need to block until the threads actually receive
the completion, as the shutdown process includes a separate step that waits
until the threads exit, and the new approach avoids an unnecessary delay.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1801635 13f79535-47bb-0310-9956-ffa450edef68