simulation: retry if we hit a collision on tcp/udp ports

[nonsense]

Due to setupListening in p2p/server.go it is possible to get a free TCP port, but for the OS to already be using the same UDP port. When that happens, Swarm crashes.

This PR is adding a retry mechanism to the simulation framework, so that when the OS gives us a TCP port, whose corresponding UDP is taken, we try to reboot the node.

func (srv *Server) setupListening() error {
	// Launch the TCP listener.
	listener, err := net.Listen("tcp", srv.ListenAddr)
	if err != nil {
		return err
	}
	laddr := listener.Addr().(*net.TCPAddr)
	srv.ListenAddr = laddr.String()

[nonsense]

@janos this is only for Exec adapter, Docker or Kubernetes don't have this problem. The general issue is that we assume that if the OS gives us a TCP port that we can bind to, then the UDP port must also be free, which is def not the case on my laptop or on Travis.