cli: make startstop_node wait 40 seconds instead of 20 for a process to go away after we signalled it to go away, before emitting a warning

Because the unit tests on the VirtualZooko? buildslave failed when it took 31 seconds for a process to go away.
Perhaps getting warning message after only 5 seconds instead of 40 seconds is desirable, and we should change the unit tests and set this back to 5, but I don't know exactly how to change the unit tests. Perhaps match this particular warning message about the shutdown taking a while and allow the code under test to pass if the only stderr that it emits is this warning.

src/allmydata/scripts/startstop_node.py

@@ -123,7 +123,7 @@ def do_stop(basedir, out=sys.stdout, err=sys.stderr):
         pass
     start = time.time()
     time.sleep(0.1)
-    wait = 20
+    wait = 40
     first_time = True
     while True:
         # poll once per second until we see the process is no longer running