Let locked accounts choose other addresses as validators

This change means that compromising validator’s key won’t necessarily mean loss of funds. While it results in a lower incentive to protect validators’ keys, it also results in a lower incentive to crack validators (as there will be little funds to steal). Locked accounts will still have an incentive to make validators secure, as compromised validators can lower the currency value.

This change also means that stakeholders will be able to easily delegate the duty of running a validator to other entity. This opens a possibility of validators being backed by multiple stakeholders.

A possible next step is lowering the entropy of validators’ drawing.