golos.stake: please add another method for delegation without interest

[gropox]

Actually transaction with delegation should be signed with both delegator and delegatee. This seems to be done, because delegatee holds all records with delegations, in order to make possible to pay interest to delegators.

But if some one delegates with interest = 0, there is no need to hold a record for such delegation. The delegatee can accumulate such delegated amouns in one record.

Thus it could be acceptable to sign delegation by just only one key of delegator, if delegation made with zero interest.

Therefore please check the possibility to check required authority of delegatee only if interest > 0