Add support for using JSONB instead of BYTEA as column type (for both journals and snapshots)

[odd]

Add a new JSON serializer trait that handles JSON serialization, e.g. something like:

trait JsonSerializer {
  def serialize(o: Any): (Json, Class[_])
  def deserialize(json: Json, targetClass: Class[_]): Any
}

Users then implement and supply (via configuration) their own JsonSerializer that handles serialization for their domain objects.