Issues with membership deletes

I've found some issues today where deleting immediate memberships does not always do the right thing in terms of deleting effective memberships. I'll add more details of the issues when I have a chance to further dive into the problems.

I've assigned this issue to the 1.4.0 release rather than the 1.3.1 release since it doesn't seem like a critical issue at the moment (nobody else has reported the problem yet). But if this needs to be resolved in 1.3.1 we can certainly aim for that.