fn: fnlb: default health state for new nodes (#508)
fn: fnlb: default health state for new nodes (#508)

* fn: fnlb: default health state for new nodes

*) Any new node now by default is in unknown state.
*) One health check is required for unknown state to move in
   to healthy/unhealthy states, then actual interval and
   thresholds apply.
*) add() no longer runs health check as this is now handled
   with the new logic.

This means during restarts fnlb will run one health check
immediately for nodes to switch to healthy/unhealthy state
to ensure speedy start, but guard against routing traffic to
unhealthy servers. After this initial state, nodes are
subjected to regular interval and thresholds.

* *) style fixes

fnproject/fn,fnproject/fn,fnproject/fn