discard stagnant messages from bounces or shunt queues
Original LP Author: Sumana Harihareswara , LP Link: https://bugs.launchpad.net/bugs/1414180
Discard messages that have been in the bounces or shunt queues for a long time.
From http://wiki.mailman.psf.io/DEV/Mailman%203.0?action=diff&rev2=30&rev1=28 -- a TODO for the Mailman 3.x series.