Internal refactor: Realm class hierachy

[cmelchior]

Part of #713

(Claus - 2) Clean up internal Realm-hierarchy. MutableRealm was originally constructed as the user-facing writable realm, but with the introduction of DynamicMutableRealm we have some common stuff across a the mutable realm and some parts of MutableRealm are not on all the actual mutable realms ... so maybe time to revisit. I tried to pull out some of the functionality into internal interfaces with default implementations in https://github.com/realm/realm-kotlin/pull/615, but there are still things to do (ex. issues highlighted in https://github.com/realm/realm-kotlin/pull/723)