gpload: exit with os._exit to prevent hang (#5335)

gpload test case will run gpload with subprocess, read stdout
and stderr from it and wait for exit. sys.exit in gpload does some
cleanup may cause deadlock between test and gpload. os._exit will
exit immediately, but we need to flush stdout and stderr before
that.

gpMgmt/bin/gpload.py

@@ -2898,4 +2898,6 @@ class gpload:
 if __name__ == '__main__':
     g = gpload(sys.argv[1:])
     g.run()
-    sys.exit(g.exitValue)
+    sys.stdout.flush()
+    sys.stderr.flush()
+    os._exit(g.exitValue)