Audit and improve database queries and table indices

I'm looking through the query log performance for quiz and noticing that some of them are surprisingly slow. Surprising until you look at the table defs and see that there's very little use of indices. E.g. quiz_node_relationship, which gets huge, doesn't have any indices.

Adding in indices is pretty easy and has almost no potential for bad side-effects. In my testing so far, adding the proper indices to quiz_choice_answer() dropped a 5 ms query down to 0.6 ms. These do add up.

Another performance problem is suboptimal queries. E.g. the DISTINCT in _quiz_get_questions() slows it down by a factor of two.