potential race condition when signals are delivered during cage exit

[qianxichen233]

Create an issue for this in case I forget about it later. It's probably of medium priority to fix, so I'll handle it once I'm done with dynamic loading if no one else has fixed it by then.

There is a time gap between when a cage starts exiting and when it is fully exited. If a signal is delivered during that time frame, it could cause issues. One ideal way to fix this is to mark the cage as dead as soon as it starts exiting, so that any signal delivered to it will immediately fail, as if the cage no longer exists.