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