Windows: Run Workers until empty task queue

Windows: Run Workers until empty task queue

Do not allow a worker thread to sleep until the task queue is
empty.  It is better for the running thread to pick up and process
a task then to sleep this thread and wait for another one to wake
up to perform the work.

Change-Id: I776bb9408ab054b045acb9bc003b88436cc4266b
Reviewed-on: http://gerrit.openafs.org/6626
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>