[bgpd] Allow accepted peers to progress even if realpeer is in Connect

[bgpd] Allow accepted peers to progress even if realpeer is in Connect

* bgpd/bgp_packet.c: (bgp_open_receive) Try fix the little race in the FSM,
  where a accept-peer that progress faster than realpeer gets closed down if
  realpeer is still just in Connect, by allowing the realpeer to be
  bgp_stop'ed and doing the regular swapping-of-FSM state.