Core API for observing group metadata
Similarly to #1447, it would be useful if the core allowed an observer to register an interest in a group's metadata and receive an initial snapshot followed by an ordered series of updates. It should be possible to register multiple observers that are ordered with respect to each other, or alternatively a single observer that specifies multiple groups to watch. This would make it easier to maintain views like the contact list, which is based on metadata from multiple groups per contact.