Implement Validator Queue

[rigelrozanski]

Looks like this should be a seperate PR

REF: #2393

Additionally, this queue should be implemented for when entire Validators are unbonded - currently there is a function in x/stake/types/validator.go named IsUnbonded() which should be removed, this function is a hacky way which we check if the validator status should be considered "unbonded" even if the actual value of Validator.Status == sdk.Unbonding. The way it's currently implemented is not ideal and can cause confusion, if we put the validators in a queue as well, and updated the status via the queue this function can go away as the Validator.Status will actually reflect the true status.