Describe the bug
go-shadowsocks2 sends TCP Keep-Alive packets every 15 seconds, on every connected and listened TCP socket (shadowsocks server-client connections, outgoing connections to other servers), which leads to high battery usage on mobile devices.

Since keep-alive interval is set also on listening socket, non-go-shadowsocks2 clients will also receive keep-alive packets every 15 seconds when connecting to go-shadowsocks2 server.

More information here: golang/go#48622

To Reproduce
Steps to reproduce the behavior:

1. Connect to go-shadowsocks2 server or run go-shadowsocks2 client.
2. Initiate TCP connection and keep it idle for some time.
3. Check keep-alive packets sending/receiving every 15 seconds with the network sniffer.

Expected behavior
I expect Keep-Alive interval to be configurable.
go-shadowsocks2 should allow to increase TCP Keep-Alive or to disable it completely.
15 seconds is a golang default value and it's way too short. For comparison, Linux sets default keep-alive value to 2 hours (which is probably too high).