DynamicValidator implementation is incomplete in mold

[HoOngEe]

next_block_proposer method of the trait ValidatorSet have assumed that validators method returns the validators sorted in a desirable order so that it can just pick the next validator by the index view.
However, the ordering rule changed after #388 and the previous behavior can not be simulated by the SimpleValidator in mold which only contains voting_power and public_key. I can copy the previous logic properly to recover the behavior.

[majecty]

The behavior is changed here: #602
We will track the problem here: #603