Add TCP Keep-Alived To Maintain Long Connections

[DuckSoft]

As is proposed by an anonymous user, mmp-go is now using a relatively short TCP read timeout when it comes to long connections, which makes SSH over mmp-go hard to use.

Here in Shadowsocks Go2 we've found a commit that should probably resolve the issue: shadowsocks/go-shadowsocks2@9dfb3e5

We can add that TCP Keep-Alive both ways in TCP connections, and hopefully this will keep the long-connection from dropping.