added db field course.cacherev

MDL-41436 Move course content cache to MUC
MDL-34397 Removing references to course.modinfo
reset cacherev and not modinfo
course overview retrieve cacherev
MDL-34397 Removing references to fields course.modinfo, sectioncache
MDL-34397 Save extra DB query since 'enablecompletion' is cached
MDL-34397 Remove ref to nonexisting course.modinfo in MNET auth
Allow user=-1 in get_fast_modinfo()