For developing the Kafka version of salobj I added a few protections to avoid interfering with the ingestion of data into the EFD from from DDS-based SAL:

• Prevent $LSST_TOPIC_SUBNAME from being "sal". That is the value wanted for production.
• Add a "kafka" field to Avro schema namespace, to make Avro names different than those used by DDS-based SAL

If and when we are ready to switch to Kafka, remove these protections.