When updating/deleting a section/module, the system now only
invalidate of the element (section/module), not the whole course cache
Also, the system now only recalculate the cache for element (section/module)
if necessary, not the whole course cache

Move module/section purging to course_modinfo:
  + course_modinfo::purge_course_section_cache_by_id was created to purge section by id
  + course_modinfo::purge_course_section_cache_by_number was created to purge section by number
  + course_modinfo::purge_course_module_cache was created to purge module