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.
MDL-41888 separate out quiz statistics calculations from report code

also code clean up.