workqueue: register and time out worker threads
workqueue: register and time out worker threads

Register and unregister worker threads.  Periodically touch heartbeat
when idle.  Set heartbeat timeout before processing a queue item.

Signed-off-by: Sage Weil <c5b6ff1fd4f0bde10db63d7a3e07166cf6c4229f@dreamhost.com>

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph