simulation: Pass MockValidators into operations

[ValarDragon]

This will enable staking message types to navigate the state space much more effectively. (There will be much less failed simulation attempts there)

The struct for this should be similar to simulation.Account. We should also consider wrapping accounts and validators into a single struct which we pass into the operations.

[cwgoes]

Hmm, they could also just take a stake.Keeper and read the validators from the context - if we use mockValidators we'll to consider block offsets due to the validator update delay.