Ensure export reports progress

If a hadoop task does not either write output or reports status for 10 minutes it is killed. Make sure we check back in periodically with hadoop to ensure it doesn't kill our export processes.

@brianhw @rocha
Not all that clean to override.  The main is hardcoded into mrrunner.py, and mrrunner.py would have to be replaced in HadoopJobRunner.run_job.  Might actually be easier to patch luigi directly than to work around....