MDL-72991 Course: Partial course cache rebuild

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

availability/condition/date/classes/condition.php

@@ -292,14 +292,16 @@ class condition extends \core_availability\condition {
                 $updatesection->availability = json_encode($tree->save());
                 $updatesection->timemodified = time();
                 $DB->update_record('course_sections', $updatesection);
+                // Invalidate the section cache by given section id.
+                \course_modinfo::purge_course_section_cache_by_id($courseid, $section->id);
 
                 $anychanged = true;
             }
         }
 
-        // Ensure course cache is cleared if required.
         if ($anychanged) {
-            rebuild_course_cache($courseid, true);
+            // Partial rebuild the sections which have been invalidated.
+            rebuild_course_cache($courseid, true, true);
         }
     }
 }