Replace calls to GetLastValidators with IterateLastValidatorPowers

[mpoke]

Problem

The GetLastValidators method of the staking module has a panic in it. Calling this from BeginBlock or EndBlock might lead to the chain to halt.

Closing criteria

All the calls to GetLastValidators are replaced with IterateLastValidatorPowers.

[MSalopek]

Would it be possible to have a utility function that returns the active portion of the validator set instead of adding IterateLastValidatorPowers in all places?

e.g.

providerKeeper.GetLastActiveValidatorPowers()

func (k *Keeper) GetLastActiveValidatorPowers() <return-type> {
  ...
  k.stakingKeeper.IterateLastValidatorPowers(...)
  return <return-type>
}