Don't subscribe to unused groups

[dankamongmen]

If we're both not tracking an object class and not invoking a callback, we ought not subscribe to its group(s), as these events will just be extra, useless work. The one exception is ifaces, which are needed as the indexing backbone of everything else.

Though really, we need to work even if we miss an iface message, so maybe other messages referencing an unknown idx ought create an iface? But we wouldn't know its name...