Speed up MagicFolder service shutdown

Also work-around a tricky, mysterious failure in the test suite by explicitly
flushing the eventual call queue.  I don't understand where the call that is
landing there comes from or why some other part of the code isn't properly
waiting on it.

src/allmydata/frontends/magic_folder.py

@@ -712,6 +712,8 @@ class Uploader(QueueMixin):
             d = self._notifier.wait_until_stopped()
         else:
             d = defer.succeed(None)
+        # Speed up shutdown
+        self._processing.cancel()
         # wait for processing loop to actually exit
         d.addCallback(lambda ign: self._processing)
         return d
@@ -1198,6 +1200,10 @@ class Downloader(QueueMixin, WriteFileMixin):
     def stop(self):
         self._log("stop")
         self._stopped = True
+
+        # Speed up shutdown
+        self._processing.cancel()
+
         d = defer.succeed(None)
         # wait for processing loop to actually exit
         d.addCallback(lambda ign: self._processing)

src/allmydata/test/test_magic_folder.py

@@ -1448,6 +1448,9 @@ class SingleMagicFolderTestMixin(MagicFolderCLITestMixin, ShouldFailMixin, Reall
         self.local_dir = os.path.join(self.basedir, u"local_dir")
         self.mkdir_nonascii(self.local_dir)
 
+        from foolscap.eventual import flushEventualQueue
+        self.addCleanup(flushEventualQueue)
+
         d = self.create_invite_join_magic_folder(self.alice_nickname, self.local_dir)
         d.addCallback(self._restart_client)
         # note: _restart_client ultimately sets self.magicfolder to not-None