MDL-41436 Move course content cache to MUC

added cache definition and DB field course.cacherev for cache validation
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()