Fix draft_backtest recommending the same algo pick across multiple rounds

algo_pick only excluded players actually taken in the real draft, so a
player it recommended in an earlier round stayed available and could be
recommended again if nobody else took him for real before your next turn
-- DJ Moore showed up in 4 different rounds in one backtest. Roster need
was also computed from your real roster instead of what the algo would
have actually rostered by that point.

Now tracks its own hypothetical picks and roster, the same way optimal
already does with optimal_taken_keys/my_taken_pos.