[muxorch] run updateroutes before disbling neighbor during update

[Ndancejic]

fdb update from an active mux port to a standby mux port when a static route was present caused orchagent crash. This is because the route was still pointing to the active neighbor nexthop when disabling this neighbor. This calls updateNexthopRoutes before disabling standby neighbor (but after neighbor nh is updated in muxorch) to set the route nexthop to the tunnel route.

What I did call updateNextHopRoutes when adding standby neighbor

Why I did it Mac move between an active and standby mux port while a route was present was causing orchagent crash because route was still pointing to active next hop.

How I verified it added vstest to cover this scenario