Cleanup on thread termination

[shirok]

As we use threads more for transient concurrency, we need to use thread-terminate! more often and it is more likely that we hit its fundamental shortcoming---the thread being terminated can't have a chance to do any cleanup.

It will be nice that at least we try to run some sort of cleanup thunks, including rewinding dynamic handlers. thread-terminate! is defined ot kill the thread without any unwinding, so we can add more graceful termination API. One thing is that dynamic handlers can invoke previously captured continuations that prevents the thread from finishing the thread thunk. We need a way to prevent that.