Improve performance for question lookup.

I have reviewed and tested this patch. It preserves functionality and improves performance tremendously. Here's the resulting change to the query with a MySQL backend:

Before: SELECT q.*, RAND() AS random_field FROM textcaptcha_questions q ORDER BY random_field ASC LIMIT 1 OFFSET 0;
After: SELECT q.* FROM textcaptcha_questions q LIMIT 1 OFFSET 0;

The new query eliminates the need for a temp table and filesorting. In testing on my local install it was roughly 495% as performant as the old query (i.e., it did the same work in 80% less time). For those interested, I calculated that like so: I ran the following code via Devel on the old code and the new.

$before = microtime(TRUE);
for ($i = 0; $i < 10000; $i++) {
  textcaptcha_get_random_question();
}
$after = microtime(TRUE);
$elapsed = $after - $before;
dpm($elapsed, 'Elapsed');

It took the old query 19.757333040237 seconds to run 10,000 times. The new query took only 3.9935119152069 seconds to do the same. That's a reduction of 15.763821125 seconds, leading to the percentages cited above.

Thanks everyone! Merged and released with 6.x-1.3 and 7.x-1.4.