add backwards compatibility for doctur with legacy behavior

[jpserrat]

Adding dctur backward compatibility with legacy behavior:

• holepuncher (svc in the new code) now switch to client on third retry.

[jpserrat]

@MarcoPolo I've added the switch in holepuncher. Just a few points:

• I didn't add a random time for the switch, I didn't understand why it is needed. Can you elaborate?
• I've added the switch only on the server (holepuncher.go) as suggested on the issue, but for both clients we have to also add this on svc.go. I was think of making a peer store to track peer attempt and switch after some attempts. Do you have any suggestions?

[MarcoPolo]

Thank you @jpserrat!

[jpserrat]

Thank you @MarcoPolo!