Allow subclasses to synthesize their properties

Commit 256aaecbf012b7935a0db697192874f39d9f7567 disables automatic synthesis of properties in subclasses of CBLModel. While I understand users who do not know to set their properties as `@dynamic` can cause confusion I believe this is an issue of education, not hampering user freedom. This PR re-enables property synthesis in CBLModel subclasses.