This ensures that graphdriver plugins can properly cleanup on daemon
exit.
Also prevents errors during shutdown when it tries to send the plugin a
`Cleanup()` request but ultimately times out since it's already been
shutdown.
Signed-off-by: Brian Goff <cpuguy83@gmail.com>
(cherry picked from commit 4b400ecc4d)
Signed-off-by: Victor Vieux <victorvieux@gmail.com>