Implement a pipeline in other runners

[Barry Warsaw]

Stephen observed that it might be useful to implement a pipeline of handlers (even chain processing?) for other runners such as archive, bounce, and command. This is probably not something for 3.0 final, thus I'm marking it triaged,low. But it's an interesting idea and may lead to other useful refactoring.

[Axel Beckert (xtaran)]

This would be very useful to use SpamAssassin.py together with spam directed to the -bounces@ addresses which seems to rise currently. Those spams are currently always forwarded to the list owner as "Uncaught bounces" and due to new, usually local mail headers usually not filtered at the owner's mail account.

[Abhilash Raj (raj-abhilash1)]

This bug has been moved to the new gitlab repo here: https://gitlab.com/mailman/mailman/issues/38