[Runtime] Remove RDF reader/writer dependency on IdentityManager
[Runtime] Change IModelIdentityManager and implement AModelChangeManager

AModelChangeManager takes an IModelIdentityManager as constructor argument. It
uses it to implement resolveSubject(..), resolveObject(..), resolveObjectUri(..)
and resolveValue(..) helper methods.

Subclasses of AModelChangeManager have to implement coerceOrInstantiate(..).