4 automatic schema registration retrieval

[stefan-cardnell-rh]

For Producing (serializing), Topic subclasses must return a schema from key_schema and value_schema, otherwise AvroSerializer will produce an error.

For Consuming (deserialzing), Topic subclasses can leave key_schema and value_schema returning None, and AvroDeserializer will fetch the right schema based on the message contents. Or they can optionally override it to use a different schema.

Also serializer_conf added as a parameter to Topic so serialization conf can be adjusted (e.g. adjusting subject.name.strategy).

[bodja]

@stefan-cardnell-rh we can deploy this one as a temporary solution. I'll work today to make it more flexible and strict.