Query Optimization

Removed GROUP BY and SUM() to further speed up the query. Results for 20000+ users are now:
Time to fetch one ranked row for current user with 100 attempts: Average: 0.000322525501251 - Min: 0.000308990478516 - Max: 0.000419855117798 - Total: 0.0322525501251
Time to fetch one page of all users with 100 attempts: Average: 2.10098520279 - Min: 2.07785606384 - Max: 2.26523208618 - Total: 210.098520279