allow MOTD to be displayed multiple times

(one displayed message per received welcome["motd"])

There's not much value in prohibiting the server from sending multiple
MOTD messages, and it would prevent us from using it to display a "your
client is using an old API, please upgrade" message after having already
sent a regular "please donate" MOTD message. (We could send a second
welcome message with ["error"] to kill the client, but ["motd"] is the
most convenient way to deliver a non-fatal warning).

src/wormhole/test/test_wormhole.py

@@ -47,10 +47,13 @@ class Welcome(unittest.TestCase):
                          [mock.call.write(u"Server (at relay_url) says:\n"
                                           " message of\n the day"),
                           mock.call.write(u"\n")])
-        # motd is only displayed once
+        # motd can be displayed multiple times
         with mock.patch("sys.stderr") as stderr2:
             w.handle_welcome({u"motd": u"second message"})
-        self.assertEqual(stderr2.method_calls, [])
+        self.assertEqual(stderr2.method_calls,
+                         [mock.call.write(u"Server (at relay_url) says:\n"
+                                          " second message"),
+                          mock.call.write(u"\n")])
 
     def test_current_version(self):
         w = wormhole._WelcomeHandler(u"relay_url", u"2.0", None)

src/wormhole/wormhole.py

@@ -178,18 +178,15 @@ class _WelcomeHandler:
     def __init__(self, url, current_version, signal_error):
         self._ws_url = url
         self._version_warning_displayed = False
-        self._motd_displayed = False
         self._current_version = current_version
         self._signal_error = signal_error
 
     def handle_welcome(self, welcome):
-        if ("motd" in welcome and
-            not self._motd_displayed):
+        if "motd" in welcome:
             motd_lines = welcome["motd"].splitlines()
             motd_formatted = "\n ".join(motd_lines)
             print("Server (at %s) says:\n %s" %
                   (self._ws_url, motd_formatted), file=sys.stderr)
-            self._motd_displayed = True
 
         # Only warn if we're running a release version (e.g. 0.0.6, not
         # 0.0.6-DISTANCE-gHASH). Only warn once.