Refactor load balance

[L-M-Sherlock]

I try to make the code more clear and efficient. @user1823, could you review this PR?

[user1823]

This refactored code is much easier to understand. Thanks.

I suggest renaming some of the variables:

1. self.due_cnt_perday_from_first_day → self.due_cnt_per_day ("from first day" is not needed IMO)
2. self.backlog → self.due_today
3. self.learned_cnt_today → self.reviewed_today