Allow AsyncEvaluator to return after a percentage of jobs are ready

[Alec Lofquist]

Do failed jobs count towards this percentage? Yes.

Fraction should be on the interval 0..1? Yes.

Pending points from previous iterations should NOT count towards the total. The evaluate_population functions will need to change to take in two arguments instead of one (second argument = pending points from previous iterations).

[Balaji Pokuri]

Fixed in ddebfed and d1c8adf