Move data directory from ontologies/update to version-specific directory ontologies/update17to18
Create ontologies/update15to16 and ontologies/update16to17 from previous update data directories.
Revise UpdateKnowledgeBase to accept the data directory as a parameter instead of having it hard-coded.
Create classes like Release18Migrator that will call UpdateKnowledgeBase with the appropriate directory, and do other release-specific migration tasks.
Make corresponding changes in startup_listeners.txt
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