The two tables the mapping writes, and the prefix of the mapping cache kept for them.
The mapping stage maps each broker's instrument snapshot into unified.instruments and unified.broker_mappings,
which bin/unified/instruments/map runs each day and the REST API reads. The names live here rather than in each
query: the writer in base.py, the index lookup in crossref.py, the duplicate merge in collisions.py, the resolver
in resolution.py and the mapping cache in cache.py read them from this module.