Clean up todos

We just decided all TODOs in the code should be linked to a tracker issue. http://docs.moodle.org/en/Development:Coding_style#TODO_comments_must_link_to_a_tracker_issue.

Of course, most of the ones we have at the moment don't.

I made a little script .../lib/simpletest/todochecker.php which lists all the todos in the code, together with whether they link to a tracker issue as per the guideline.

Before the Moodle 2.0 release, we should use that to try to fix as many todos as possible, and ensure that the rest have corresponding tracker issues filed.