HHH-5920: improve the performance of PersistentClass

This implements most of HHH-5920. Specifically:
- new transient lazily instantiated properties (mappedClass, proxyInterface)
- minor improvement to performance of getProperty(String propertyName, Iterator iterator)
- tests to ensure the methods still behave as expected

The extra complexity to have a no exception version of getProperty(String propertyName, Iterator iterator) was seen as overkill.