MDL-41436 implement new increment_revision_number() function

This is intended as a revision counter for local modinfo caching. It should be a lot more reliable and faster than doing it in PHP. Please note you need to invalidate all course caches after using this.
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
MDL-41436 Additions