Implement Validator Commission Model

[rigelrozanski]

Validator commission change boundary logic is currently unimplemented

• first verify and update the spec

[alexanderbez]

@rigelrozanski can you point me to the spec?

[rigelrozanski]

https://github.com/cosmos/cosmos-sdk/blob/develop/docs/spec/staking/transactions.md#txeditvalidator

rateChange isn't spec'd - it's just an implied calculation