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.