StorableTypes in assemblies loaded dynamically are not registered correctly

[gkronber]

.NET loads assemblies dynamically on demand. In HEAL.Attic we discover and cache all types marked with the StorableType attribute when the ProtobufSerializer is first used.

This means that we currently do not register StorableTypes correctly if they are located within assemblies which are loaded at a later point in time.

[jkarder]

Done