MDL-59067 analytics: Store indicator calculations

This was supposed to be split into multiple commits to make it easier to understand
but I failed to do it properly. So this is the list of changes:

- New analytics_indicator_calc db table to store indicators calculations
- Reuse previous calculations during prediction/training; other models
  previous calculations should also be reused as long as they belong to
  the same sample (sampleid depends on sampleorigin), time range and indicator
- Allow bulk inserting of these calculations as this can hurt database performance
- Block the same analysable to be analysed for training and for prediction
- Use a new instance of the target and use it for is_valid_* functions
  as using ::is_valid_sample can lead to problems if people
  uses it to cache stuff

analytics/classes/dataset_manager.php

@@ -119,6 +119,9 @@ class dataset_manager {
      * @return bool Could we get the lock or not.
      */
     public function init_process() {
+
+        // Do not include $this->includetarget as we don't want the same analysable to be analysed for training
+        // and prediction at the same time.
         $lockkey = 'modelid:' . $this->modelid . '-analysableid:' . $this->analysableid .
             '-timesplitting:' . self::clean_time_splitting_id($this->timesplittingid);