two small changes:
flush outgoing heartbeats
Prevents mismatch between heartbeat timer and actual heartbeat send/recv events.
Applied in notebook and parallel.
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).
This looks good - I think it can be merged.