common/Finisher: allow finisher to be restarted

Allow a start() after stop() by clearing the stop flag on a successful
stop.

Signed-off-by: Sage Weil <sage@inktank.com>
os/FileStore: allow mount after umount

Clear the stop flag on successful shutdown so that we can start again
later.

Signed-off-by: Sage Weil <sage@inktank.com>
common/WorkQueue: allow start() after stop()

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #891 from ceph/wip-filestore-remount

allow various threaded services to be restarted to allow mount->umount->mount to work

Reviewed-by: Greg Farnum <greg@inktank.com>