Caps for list member count and outgoing messages per time slice

Original LP Author: Sumana Harihareswara , LP Link: https://bugs.launchpad.net/bugs/1414170

Caps for list member count and outgoing messages per time slice.

From http://wiki.mailman.psf.io/DEV/Mailman%203.0?action=diff&rev2=30&rev1=28 -- a TODO for the Mailman 3.x series.