Fix listen_for_events() in Client

[Arjentix]

When running some integration tests that use event subscription (for example time_trigger_execution_count_error_should_be_less_than_10_percent) an error message apears in log:

ERROR iroha::torii::routing: Failed to subscribe someone error=Event consuming error: WebSocket error: WebSocket protocol error: Connection reset without closing handshake

So Client uses API a bit wrong and doesn't close subscription connection properly. To make closing handshake a WebSocket::close() method should be called on stream field of EventIterator.

Ideally this fix should provide the same good-looking API as before. User shouldn't close connection manualy