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).

IPython/parallel/controller/heartmonitor.py

@@ -64,7 +64,7 b' class HeartMonitor(LoggingConfigurable):'
     pongstream: an XREP stream
     period: the period of the heartbeat in milliseconds"""
 
-    period = Integer(1000, config=True,
+    period = Integer(3000, config=True,
         help='The frequency at which the Hub pings the engines for heartbeats '
         '(in ms)',
     )