Restart slave.jar when it dies.

Summary:
Sometimes slave.jar dies due to an OOM error or the like.  In those
cases, make it automatically restart.  Jenkins doesn't notice that
slaves are dead in order to restart it themselves.

Test Plan:
Fingers crossed.  I'll install this on jenkins, terminate all the
jenkins-slave machines on ec2, and run make-allcheck, and make sure
the slaves start up properly.  I don't know what
com.trilead.ssh2.Connection.execCommand does, but it looks like it
just runs the given command in the shell, based on code example #6 at
   http://www.programcreek.com/java-api-examples/index.php?api=com.trilead.ssh2.Session
   >>> sess.execCommand("uname -a && date && uptime && who");

Reviewers: jlfwong

Reviewed By: jlfwong

Subscribers: chris

Differential Revision: http://phabricator.khanacademy.org/D14010