HttpServerConnection: use AsioTlsStream::GracefulDisconnect()

This new helper function has proper timeout handling which was missing here.

Author: julianbrost

lib/remote/httpserverconnection.cpp

@@ -95,9 +95,7 @@ void HttpServerConnection::Disconnect()
 
 			m_Stream->lowest_layer().cancel(ec);
 
-			m_Stream->next_layer().async_shutdown(yc[ec]);
-
-			m_Stream->lowest_layer().shutdown(m_Stream->lowest_layer().shutdown_both, ec);
+			m_Stream->GracefulDisconnect(m_IoStrand, yc);
 
 			auto listener (ApiListener::GetInstance());