[SPARK-11655][CORE] Fix deadlock in handling of launcher stop().

The stop() callback was trying to close the launcher connection in the
same thread that handles connection data, which ended up causing a
deadlock. So avoid that by dispatching the stop() request in its own
thread.

On top of that, add some exception safety to a few parts of the code,
and use "destroyForcibly" from Java 8 if it's available, to force
kill the child process. The flip side is that "kill()" may not actually
work if running Java 7.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #9633 from vanzin/SPARK-11655.