fun <T> Observable<T>.notUsed(): Unit
This function should be invoked on any unwanted Observables returned from RPC to release the server resources.
subscribe({}, {}) was used instead of simply calling subscribe() because if an {@code onError} emission arrives (eg. due to an non-correct transaction, such as 'Not sufficient funds') then {@link OnErrorNotImplementedException} is thrown. As we won't handle exceptions from unused Observables, empty inputs are used to subscribe({}, {}).