Fix already bound assert for extensions

[slavam2605]

There is a following scenario:

1. SomeModel: RdExtBase is created on one side of a protocol
2. On the other side 1ProtocolExtListenerManager1 creates and binds this extension
3. In RdExtBase.init > Protocol.submitExtCreated all listeners are invoked
4. One of the listeners invokes SomeModel.getOrCreateExtension("mySubModel")
5. This sub-extension is created and bound
6. When all listeners are invoked, RdExtBase.init continues and tries to bind all its bindableChildren
7. Sub-extension is already there and is already bound => we get a failed assert about binding already bound model

To fix this case, we can check that an extension is already bound and bound to us as a parent. Then we have no need to bind it again.