Merge pull request #1312 from minrk/hb

two small heartbeat changes:

* flush outgoing heartbeats (notebook and parallel):
  - Prevents mismatch between heartbeat timer and actual heartbeat send/recv events.

* relax default heartbeat period in IPython.parallel to 3s from 1s
  - Matches heartbeats elsewhere.  3s should be extremely conservative, as most heartbeat responses are O(1ms) unless there's a bug in our code (like the above flush and previously-discovered GIL issues).

closes #1304

IPython/frontend/html/notebook/handlers.py

@@ -481,6 +481,8 @@ class IOPubHandler(AuthenticatedZMQStreamHandler):
                 if self._kernel_alive:
                     self._kernel_alive = False
                     self.hb_stream.send(b'ping')
+                    # flush stream to force immediate socket send
+                    self.hb_stream.flush()
                 else:
                     try:
                         callback()