MDL-41754 (1) Progress tracking : moved from backup and restore to core
MDL-41754 (2) quiz statistics : using Sam's new progress classes
MDL-41754 (3) quiz statistics : need to escape core_php_time_limit

after MDL-42084 was merged into upstream, introducing
core_php_time_limit, this branch that moved a reference to
core_php_time_limit to a new namespace was broken.
MDL-43027 core progress : new method keeps track of progress internally
MDL-41756 (1) quiz statistics : use new method increment_progress

to cut down on clutter before adding new break down by variant stuff
MDL-41756 (2) quiz statistics : break down question stats by variant
MDL-41757 quiz statistics : limit subq and variant stat rows

and print min, median and max.