Delayed expunge / undeleting messages

[Rob Mueller]

https://bugzilla.andrew.cmu.edu/show_bug.cgi?id=2871

> I also found out that delayed_expunge doesn't help if a whole folder got
> deleted. So if something like your proposed solution gets included that
> would really be a very nice feature.

Agreed. This would also allow for "complete" backups of all emails and all 
folders. Currently there's a window of emails uploaded/delivered between the 
last backup and a folder being deleted that can never be restored.

I know David Carter implemented a patch for this in 2.2. I'm not sure if 
Wesley's patch is related to it or a completely separate implementation.

Rob