Key rotation helper method

[robrigo]

It would be helpful and improve account security, if there was a command available in the wallet to rotate account keys for any combination of labels.

Perhaps the validation is simply proving that you hold the current key for a given label, or a key with a higher permission for that account.

I'm pretty sure this is possible on Steem, not sure if Peerplays needs to be modified at the blockchain level to support this.

[robrigo]

I'm shooting from the hip, but maybe this is possible using the transaction builder + account_update_operation.

https://github.com/PBSA/peerplays/blob/master/libraries/chain/include/graphene/chain/protocol/account.hpp#L120