MDL-45803 mod_quiz: added attempt snapshot for the attempt and preview started events
MDL-45803 mod_quiz: prevented the attempt_started event from being triggered twice
MDL-45803 mod_quiz: removed usage of the function quiz_fire_attempt_started_event
MDL-45803 mod_quiz: removed the quiz_fire_attempt_started function

Also commented in mod/quiz/upgrade.txt to inform developers.