bc_http: Enhance 'always-on' mode:

- Throttle the reconnection attempt when the nodelist has been cycled
- Don't reconnect when no configuration exists (if the nodelist has
  ended)

Change-Id: Iade455ceb9ef2cc5e3c2e0e81ac99f2e6bc9d289
Reviewed-on: http://review.couchbase.org/38704
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>