Add --list parameter to showJobs.php

For listing all of the jobs in the queue. Post-processing will enable
most sorts of analysis that you would want to do. When we used SQL, this
wasn't reallly necessary, but getting jobs out of Redis is a bit more
tedious.

The format is similar to the log made by runJobs.php.

Change-Id: Idfa3f52ba89638892fc7a64d6566955d66de5958
(cherry picked from commit bc16409852db4c2f8db68d3db07d2c9cc612ea6b)