[FR] Server instances per local port

So, multiple server is a great addition to shadowsocks, however my issue is that it essentially works as load balancer rather than actual being separate servers. It would be great if we could define for each server a local listener instance with unique local port. For example, Server1-3 declared in the config file gets requests from 127.0.0.1:1080 but Servers 3-5 will be getting requests from 127.0.0.1:1081. This mean sslocal can open multiple listen ports and two or more instances no longer required for this to work.

So essentially:

{
    "servers": [
        {
            "address": ".ip.ip.ip.ip",
            "port": 8388,
            "password": "hello-world",
            "method": "aes-256-gcm",
            "timeout": 7200,
            "local_port": 1081,
            "local_address": "127.0.0.1"
        },
        {
            "address": "ip.ip.ip.ip",
            "port": 8389,
            "password": "hello-kitty",
            "method": "chacha20-ietf-poly1305",
            "local_port": 1082,
            "local_address": "127.0.0.1"
        },
    ],
    #Global listener for all servers who dont have it defined per server
    "local_port": 8888,
    "local_address": "127.0.0.1"
}

Start by examining sslocal's configuration and listener setup, using the per-server and global local_port/local_address example as the desired behavior. Done means servers can be grouped by distinct local listeners, while servers without per-server settings use the global listener, without requiring multiple sslocal instances.