Transaction receipt should be signed by the operator.

[smartcontracts]

Is your feature request related to a problem? Please describe.
Transaction receipts are supposed to be signed by the operator to ensure the user that the operator has actually received their transaction. However, currently the operator just sends back the same encoded transaction that was originally sent.

Describe the solution you'd like
The operator should sign the transaction (perhaps hash of the hash of the transaction to avoid tricking the operator into signing a send...) before sending it back to the client.

[Jinglan]

Addressed in #48