The problem was a thread handle resource leakage when creating
threads for handling connections on Windows. This could lead
to Windows servers eventually running out of handles.

This was a regression introduced in 5.7 by WL#6407. In this
WL we added support for joining threads on Windows in order
to do proper thread cleanup during shutdown. In order to
do this we had to keep the thread handle open as it is
needed for joining the thread.

The bug was that the handle was kept open also for threads
that are detached. Such threads are never joined and thus
the handle was not closed until the server terminated.
The (by far) most common type of thread which is detached,
is threads for handling connections.

This patch fixes the problem by closing thread handles
right after thread creation if the thread is created as
a detached thread.