Windows: RDRLib Worker Thread shutdown

The thread waiting to unload the library is polling the state of
the AFS_WORKER_INITIALIZED flag in the PoolContext->State field
for each worker.  Ensure that the thread performs no actions other than
PsTerminateSystemThread() after clearing the AFS_WORKER_INITIALIZED flag.

Change-Id: I0baa92f05a5d5f00219a2cc63ca71c7a6a1f52b5
Reviewed-on: http://gerrit.openafs.org/10383
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>