Determine IP version on the basis of the source address

This allows to "tunnel" IPv6 connections in IPv4 connections between
haproxy and go-mmproxy.

tcp.go

@@ -51,7 +51,7 @@ func tcpHandleConnection(conn net.Conn, logger *zap.Logger) {
 	}
 
 	targetAddr := Opts.TargetAddr6
-	if AddrVersion(conn.RemoteAddr()) == 4 {
+	if AddrVersion(saddr) == 4 {
 		targetAddr = Opts.TargetAddr4
 	}