Handle get_memberships() for other roles

[Barry Warsaw]

This is a placeholder bug for work that has to be done. It may not be important enough for the 3.0 release, but let's make sure it happens early in 3.1.

Members can subscribe to a mailing list either via an explicit address or via their user's preferred address. They can also have two membership records for the same role, using both techniques. IRoster.get_member() isn't able to represent this, so I added IRoster.get_memberships(). I fixed the queries for MemberRoster, and updated their implementations to match the interface.

This needs to be done for the other roster implementation classes.

[Abhilash Raj (raj-abhilash1)]

This bug has been moved to the new gitlab repo here: https://gitlab.com/mailman/mailman/issues/18