message: Warn when using tokio DNS resolver
  Fixes
diff: use std::{
  io::{self, Error, ErrorKind},
  net::SocketAddr,
  + sync::atomic::{AtomicBool, Ordering},
  };
  +use log::warn;
  use tokio::net::lookup_host;
  use crate::context::Context;
  /// Perform a DNS resolution
  pub async fn resolve(_: &Context, addr: &str, port: u16) -> io::Result<Vec<SocketAddr>> {
  + static TOKIO_USED: AtomicBool = AtomicBool::new(false);
  + if !TOKIO_USED.swap(true, Ordering::Relaxed) {
  + warn!("Tokio resolver is used. Performance might deteriorate.");
  + }
  +
  match lookup_host((addr, port)).await {
  Ok(v) => Ok(v.collect()),
  Err(err) => {
  