Maintenance Script that Can Dump Queue Messages

We need a script that can remove, with a backup, all the messages
in a queue. This is that. (We need it because adyen used to be
very messy with the limbo queue and it seems to have gummed up
the works.)

Change-Id: Ia2b6408ae67be5275ff37120fc9d9ca64902a3b0
Merge "Maintenance Script that Can Dump Queue Messages"