pub fn set_udp_bind_addr(&mut self, a: ServerAddr) {
        self.udp_bind_addr = Some(a);
    }
UDP server's bind address

* If `mode` is `tcp_only`, then it will still return this address for `UDP_ASSOCIATE` command
* Otherwise, UDP relay will bind to this address
crates/shadowsocks-service/src/local/socks/server/mod.rs