Make WriteRunningTx conform to DataBroker API

Transaction implementation (WriteRunningTx) for netconf devices, that support only writable-running capability does not conform to DataBroker API and differs in behavior from other two implementations (WriteCandidateRunningTx and WriteCandidateTx).

Namely, write operations are instantly reflected on device. Submit and cancel does virtually nothing. Write operations should be local to the transaction and not processed until submit is called.