Refine closing of streaming writer (#3394)

Author: faysou

nautilus_trader/system/kernel.py

@@ -1438,4 +1438,4 @@ def _cancel_timers(self) -> None:
 
     def _flush_writer(self) -> None:
         if self._writer is not None:
-            self._writer.flush()
+            self._writer.close()