Terminate correct tasks when a slave disconnects.

Previously, when a slave disconnected all tasks for that framework would
be removed and it would result in a bad state for a given framework.  In
the case of Hadoop, it would result in a bunch of zombie tasks running
on the slaves which never terminate.
Suicide executors if tasks don't launch.

We'll wait for 60 seconds for a task to launch once an executor has
been launched.  If no task is launched, the executor will end its own
life.