khan: cleanup
khan: cleanup

- only store metadata in the persistent map. just enough to support
  (eventual) thread cancellation and output mark lookup.

- try to delete thread state at other failure points not covered by
  %kick.

- reflect back the passed output mark rather than form.dais. not sure
  about this one yet.

ngzax/urbit,ngzax/urbit,urbit/urbit,urbit/urbit,ngzax/urbit,urbit/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,urbit/urbit,urbit/urbit,urbit/urbit,ngzax/urbit,ngzax/urbit