Handle shutting down instances in the "Unknown" state better

[townsend2010]

Currently, when trying to shut down instances that are the Unknown state, they often times won't shut down and leave the daemon in a bad state. We should be pragmatic about this state where we first try to shut down gracefully and if it doesn't after a certain time, then forcefully stop the instance.

This is particularly needed now that instance modification is possible and it's possible to overcommit resources to an instance which leaves the instance unbootable and in an Unknown state. The instance needs to be able to get to the Stopped state so the instance can be re-modified to fix to overcommitted resource.