Multi-threaded usage of non thread safe java.util.HashMap in ClassNameUtil

org.activiti.engine.impl.util.ClassNameUtil uses java.util.HashMap to cache class names.

The usage of the cachedNames hash map is NOT synchronized while this utility class might be used by more than one threads at the same time.

This can cause severe issues: the engine might fail to initialize properly or unexpected exceptions might be raised randomly later at runtime.