receive tasks, even when no engines are registered

Previously, tasks submitted when no engines were registered were
left in the upstream ZMQ queue.  This prevented the tasks being
entered into the Hub's database.

IPython/parallel/controller/hub.py

@@ -457,7 +457,7 b' class Hub(SessionFactory):'
     def dispatch_monitor_traffic(self, msg):
         """all ME and Task queue messages come through here, as well as
         IOPub traffic."""
-        self.log.debug("monitor traffic: %r", msg[:2])
+        self.log.debug("monitor traffic: %r", msg[0])
         switch = msg[0]
         try:
             idents, msg = self.session.feed_identities(msg[1:])

IPython/parallel/controller/scheduler.py

@@ -191,6 +191,8 b' class TaskScheduler(SessionFactory):'
 
     def start(self):
         self.engine_stream.on_recv(self.dispatch_result, copy=False)
+        self.client_stream.on_recv(self.dispatch_submission, copy=False)
+
         self._notification_handlers = dict(
             registration_notification = self._register_engine,
             unregistration_notification = self._unregister_engine
@@ -247,8 +249,7 b' class TaskScheduler(SessionFactory):'
         self.completed[uid] = set()
         self.failed[uid] = set()
         self.pending[uid] = {}
-        if len(self.targets) == 1:
-            self.resume_receiving()
+
         # rescan the graph:
         self.update_graph(None)
 
@@ -256,7 +257,7 b' class TaskScheduler(SessionFactory):'
         """Existing engine with ident `uid` became unavailable."""
         if len(self.targets) == 1:
             # this was our only engine
-            self.stop_receiving()
+            pass
 
         # handle any potentially finished tasks:
         self.engine_stream.flush()
@@ -439,6 +440,11 b' class TaskScheduler(SessionFactory):'
 
     def maybe_run(self, msg_id, raw_msg, targets, after, follow, timeout):
         """check location dependencies, and run if they are met."""
+        self.log.debug("Attempting to assign task %s", msg_id)
+        if not self.targets:
+            # no engines, definitely can't run
+            return False
+        
         blacklist = self.blacklist.setdefault(msg_id, set())
         if follow or targets or blacklist or self.hwm:
             # we need a can_run filter