Cosmos v0.46 upgrade follow ups

[rhuairahrighairidh]

minor fixes from code review

• noticed there's a new rename key command
• the test for RewardIndexes.Mul wasn't calling Mul
  • This (perf: add inplace decimal operations cosmos/cosmos-sdk#11004) change in sdk.Dec math introduced a change to the Mul function.
  • Previously sdk.OneDec().Mul(sdk.Zerodec()) would be exactly equal to sdk.ZeroDec()
  • with sdk.ZeroDec() being sdk.Dec{&big.Int{neg: false, abs: nil}}, eg an empty big.Int value.
  • However now the result is not exactly equal to sdk.ZeroDec() it's now sdk.Dec{&big.Int{neg: false, abs: nat{}}}, eg the nat slice inside the big.Int has been initialised.
  • This shouldn't change any of our calculations as they don't differentiate between initialised and uninitialised values.
• registered all kava msgs on the authz codec. This is to ensure authz.MsgExec signatures are only valid for the contained msg type. See https://kava-labs.atlassian.net/wiki/spaces/ENG/pages/1237483521/Legacy+Amino+Codec+Registration