[RLMSchema] Don't iterate through runtime classes unnecessarily

[ajjnix]

In my application, there are quite a few crashes with an unclear stack trace (see the screenshot). During my investigation, I found out what is happening:

1. We add a class to Realm.
2. We release a version.
3. We remove the class from the application's source code.
4. In the migration, we call deleteData(forType:). Yes, I am aware of the crosslink issue; this is not that situation. I will look into that MR later with my experience on how to fix the deletion problem.
5. deleteData(forType:) tries to determine whether to clear or delete the table.
6. To do this, it calls -schemaForClassName (RLMSchema).
7. I have explicitly defined all classes (in objectSchemaByName after +(instancetype)schemaWithObjectClasses), and none of them includes the one I'm deleting, which makes sense.
8. But it seems excessive that a search is performed in +classForString (RLMSchema), which then goes to RLMRegisterClassLocalNames and iterates through all classes. And I remind you that I have explicitly specified the ones I am using.
9. We end up in RLMIsObjectSubclass.
10. And already in class_getSuperclass, something goes wrong.

I understand that it would be good to figure out which specific class is causing this issue, but I don't have that capability. However, it seems that this MR will eliminate the need to call class_getSuperclass unnecessarily.

[tgoyne]

This will break initializing unmanaged objects prior to opening a Realm. Creating unmanaged objects using the value: initializer lazily loads the schema for that specific class, so the second object type created will see non-empty _objectSchemaByName but the class's schema won't be present.

The correct fix here is that RLMSchema needs a separate method which checks for the existence of an ObjectSchema without trying to create it if it's missing, which deleteDataForClassName: would use.