Validators updated to zero power should be unbonded (deleted from the power store)

[cwgoes]

Presently, if a validator is set to zero power - through slashing, for example - but not through the unbond command (which does perform this check), the validator will be left in the power store with zero power (and possibly passed to Tendermint, if zero power would still put it in the top 100 validators).

Demonstrative failing testcase in #1514.

[cwgoes]

cc @rigelrozanski Do you think this check should be put in slashing (checking whenever we slash a validator) or in staking (checking in stakeKeeper.UpdateValidator)?

[cwgoes]

We can see two (!) such validators at the last block of gaia-6002 - http://gaia-6002.coinculture.net:46657/validators?height=721617.

[rigelrozanski]

I'd recommend that this check be added to staking and not slashing - although the problem may be triggered through slashing, fundamentally, staking is the interface to Tendermint and thus shouldn't allow these kinds of behaviours

[cwgoes]

I'd recommend that this check be added to staking and not slashing - although the problem may be triggered through slashing, fundamentally, staking is the interface to Tendermint and thus shouldn't allow these kinds of behaviours

Agreed.

edit: I think we can put it in the .Slash() method though.