Windows: Stop the thundering herd

Windows: Stop the thundering herd

The afs redirector used notification events to wake up worker
threads when a task was added to a work queue.  Notification
events when signalled wake up all threads instead of just one.

Instead, use synchronization events to wake up a single thread at
a time and restructure the code to permit workers to wake up
additional workers if there is additional work to be performed
or during library shutdown.

Thanks to Peter Scott for his assistance.

Change-Id: I0fb9d8578035f606f03170622fc9c50a1dbfee3a
Reviewed-on: http://gerrit.openafs.org/6595
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>