DATACMNS-384 - Polished toString() representation of Version.

Trailing zeros now get removed up until the minor version number.
DATACMNS-390 - Create defensive copy for persistent entities in AbstractMappingContext.

To prevent ConcurrentModificationExceptions when iterating over PersistentEntities while further entity types are potentially added to the MappingContext we now return a immutable defensive copy of the current entity set.