Somehow fix the killing of actors in Java

Somehow fix the killing of actors in Java

Things are somehow fixed, as all tests seem to pass, but the situation
is still very messy after this commit. Contents:

- Reimplement ContextJava as subclass of ContextThread to reduce duplication.
- Don't send the StopRequest exception on host failure if we are in
  Java because *some* of the actors don't catch it well, resulting in
  simulation failure.
- Forcefully kill the process ("exit(0)" in C) after MSG_run() because
  dead actors are sometimes not completely killed, preventing the
  simulation from ending.

See the comment in ActorImpl for a better understanding of this mess
and how to fix it in the future.