Force the JVM to exit as gremlin.sh's main returns

This is a new approach to avoiding gremlin.sh hangs after quit/exit by
Titan's embedded Cassandra instance leaving non-daemon background
threads running.  CassandraDaemonWrapper no longer attempts to stop
CassandraDaemon, which doesn't kill the all of the non-daemon threads
anyway.  Instead, gremlin.sh's main calls System.exit right before
returning, forcing JVM shutdown, which, among other things, fires the
shutdown hook registered by Cassandra during startup and terminates
all of its non-daemon background threads.

This is not ideal, since it's possible that the shutdown hook will not
run or that the process will prematurely terminate before the hook
completes, in which case Cassandra may lose writes which haven't yet
been synced to the commitlog (in periodic-sync mode).

However, it's still preferable to the alternatives I've found so far.
CassandraDaemon.stop does not actually kill all of the background
threads, and my previous approach to this problem in
CassandraDaemonWrapper was essentially just a defective version of the
shutdown hook installed by Cassandra at startup (although it did have
the benefit of running without shutdown hook time and resource
constraints).

This seems to behave both in titan-cassandra unit tests and the
titan-dist integration tests, so I'm going to give it a shot on
titan05.