Database queries take too much time when loading the task list

[davidchern]

For the single list page of Task model in Admin, it takes 300+ queries for fetching query set.

I have eliminated a few columns and make some query optimizations, then the number of queries drops to only 11. Now it's smooth to load the list page.

I wish subsequently this project can have some optimization.

[jedie]

PRs are welcome ;)