Split the notifications into DidChange and RefreshNeeded

An alternative to #808, as I think DidChange might actually be the more useful one (for updating non-persisted state based on persisted data). I'm not sure I'm a fan of the name RLMRealmWillChangeNotification, and the semantics are kind of goofy (but a bit less goofy than the current semantics).

Changelog and tests haven't been updated for this yet.

@alazier @jpsim 
The realm in the notification block and the realm here are the same realm. There isn't really anything to test about the foreground realm outside the dispatch_async block that isn't tested a million other places.