MDL-41725 move db tables from quiz stats report

to question bank
move cron to clean up old cache records
move code and rename classes
further review of the quiz reports statistics code
starting to separate calculations of quiz stats,
question stats and response analysis
introduce hashcode db field for cached stats
convert code to use qubaids hashcode for caches

We just drop the old tables (including previous
upgrade steps) and re-create the new ones,
because these tables just cache calculated
statistics. No important data is stored in them.