Update message serialization used by Kafka

[emetsger]

Messages sent to Kafka are simply byte arrays with some additional metadata. The producers and consumers of messages are responsible for serializing and deserializing the byte arrays according to their needs.

RMap is using the built-in JVM Object serialization. While this is a pragmatic, short-term solution, it is not a long-term viable option. More robust serialization ought to be considered. Some specific options are:

• RDF (use the native RDF serializations supported by RMap)
• Apache Avro (provides for robust serialization and schema evolution for messages)
• XStream (xml, json)
• ...?