MDL-41436 Move course content cache to MUC

added cache definition and DB field course.cacherev for cache validation
MDL-41436 Removing references to fields course.modinfo, sectioncache

Also add the field cacherev to the lists of default fields in course retrieval functions
MDL-41436 Save extra DB query since 'enablecompletion' is cached
MDL-41436 Allow user=-1 in get_fast_modinfo()

Remove ref to nonexisting course.modinfo in MNET auth, use get_fast_modinfo() for user=-1 instead
MDL-41436 Remove DB fields course.modinfo and sectioncache