Windows: cm_QueueBKGRequest improvements

Windows: cm_QueueBKGRequest improvements

Do not add duplicate requests into the queue.  Outstanding extent requests
will be re-issued by the afs redirector on a periodic basis while
waiting for them to be satisfied.  If they are pending there is no
need to remember them a second time.

Use separate queues for Fetch and Store operations.  Store operations
might be blocked on the file server but a Fetch operation might be served
from the cache.

Change-Id: I8e15c71275921c9a062944d52bf97a0ba68fe430
Reviewed-on: http://gerrit.openafs.org/7312
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>