Engine API: make preparePayload return an object

[mkalinin]

What's done?

The engine_preparePayload return type is changed from payloadId|Error to Object|Error for the sake of the future compatibility, i.e. this change allows for adding more data to the response of this method preserving backward compatibility.

[mkalinin]

Also, do we want engine_getPayload call accept a request Object rather than payloadId?

[lightclient]

I think just payloadId is acceptable, since JSON-RPC 2.0 already accepts an arbitrary number of parameters by either position or name.

[pomnt]

What's done?

The engine_preparePayload return type is changed from payloadId|Error to Object|Error for the sake of the future compatibility, i.e. this change allows for adding more data to the response of this method preserving backward compatibility.