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>
Upstream-commit: 4b400ecc4df4bcf245f1f751a7fd0c9c193d9dbf
Component: engine