QPID-5428: Heartbeats not in use when attempting to connect with python client.

Heartbeats ignored when opening a connection, could hang indefinitely
Need to cover 3 cases (test included):
- Connect sucessful but then broker stalls.
- Connect to a stalled broker that never responds.
- Fail-over to a stalled broker that never responds

All cases are handled by the following fixes to driver.py:
- Check for heartbeats even before engine._connected since we may time out
  before receiving open-ok if the peer is stalled and never sends data.
- Set _last_in and _last_out so that we time heartbeats from the start of the
  connection if no data is ever sent or received.
- Call self.update_status in Driver.timeout to detect connection closed due to
  heartbeat timeout (rather than a readable or writeable event.)
  Make update_status a no-op if engine or transport are not yet set up.
- Don't consider reconnect complete in connect(), wait till we get the open-ok.
  See the comment on Driver._check_retry_ok()

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1556971 13f79535-47bb-0310-9956-ffa450edef68