MVELDialectRuntimeData is not thread safe

MVELDialectRuntimeData#mvelReaders is accessed concurrently but is not thread safe due to the usage of a HashSet. This may cause lost updates. (See ForkJoinPool-1-worker-1.txt and main.txt )

If this happens and the kie base is serialized and deserialized it results in a NullPointerException at runtime. (See NPE.txt )

We observed that the entries in this Set (mvelReaders) are serialized and important for the deserialization step because they contain information about transient objects that have to be re-initialized after deserialization.
While losing this information due to concurrent access does not cause problems during compilation, serialization and deserialization of a kie base, they may cause NullPointerExceptions at runtime when transient fields remain uninitialized.