LocalAnnounce: connection with different gateway

@tangdou1 said [here](https://github.com/HelloZeroNet/ZeroNet/pull/1592):
> you can connect others peers if you know their ip and port by
> http://127.0.0.1:43110/1HeLLo4uzjaLetFx6NH3PMwFP3qbRbTf3D/?zeronet_peers=ip:port
>
> It can also be used in the condition that two users in different gateway but in one big local network which means they can ping each other. Now, the automatic local peers discover can only be used in the same gateway. However, this method can expand its scope.

Start by tracing the automatic local peer discovery described in the issue and compare it with the `/1HeLLo4uzjaLetFx6NH3PMwFP3qbRbTf3D/` endpoint's `zeronet_peers=ip:port` behavior. Done means peers in different gateways on the same local network can be discovered automatically, with coverage for the changed behavior.

索引モデルが issue の本文から書いたものです。