Use Infinispan externalizers to reduce serialized size of replicated data

Testing indicates that serialized web sessions and SFSB are much bigger in AS7 than in EAP5. This is largely due to the increased size of the class descriptors which additionally specify module names. While ClassTables have been written for the contents of the MarshalledValues which are stored in the cache, the same optimizations should happen for the cache keys and marshalled values themselves.