OFCONNECTIONMANAGER_CONFIG_ECHO_OPTIMIZATION

Reviewer: @dtalayco

Allow compile-time specification of echo generation behavior.

- OFCONNECTIONMANAGER_CONFIG_ECHO_OPTIMIZATION == 1
  Echo requests are only sent when the controller connection is idle.
  (This was the existing behavior).

- OFCONNECTIONMANAGER_CONFIG_ECHO_OPTIMIZATION == 0
  Echo requests are generated every 10 seconds, regardless of other activity.

  This avoids long periods of silence between the switch and controller when
  when the switch is busy processing messages which generate no replies.