Add support for slowly changing mapping tables

## Background
Streaming conformance currently uses static mapping tables read from HDFS.
If a job is running long enough the mapping tables become outdated.

## Feature
Long-running streaming conformance jobs need a way to make in-memory mapping tables up to date.

Not sure yet how to pick up the changes to mapping tables.
- [ ] Periodically check the HDFS folder with mapping tables for updates
- [ ] Subscribe to a Kafka topic with changes to mapping tables

Which way to go depends on how MDM and MDR are done.