Performance optimization: Don't load all attempts on start

[muqiuq]

If a quiz attempt is started, studentquiz has to make number of questions times 2 inserts into the db. This can take a lot of time until the quiz starts. If the quiz is not finished, there are a lot of unused data in the database.
This should be changed. The question_attempts and question_attempt_steps db entries should inserted, when the specific question is beeing loaded.

[frankkoch]

Just to clarify: if a quiz is started, StudentQuiz used to prepare the quiz by inserting 2 records for each question into the database. In case of a huge set of questions, this caused slow loading of the quiz. Also, this data remained in the db, even if the quiz was not completed.

This performance optimization makes inserts for a specific question only once it's loaded, not for all questions at the start of a quiz.