refactor(server): work around deprecation of poll_accept method in tokio (#1890)

src/common/drain.rs

@@ -3,6 +3,7 @@ use std::mem;
 use tokio_sync::{mpsc, watch};
 
 use super::{Future, Never, Poll, Pin, task};
+use futures_util::FutureExt as _;
 
 // Sentinel value signaling that the watch is still open
 enum Action {
@@ -99,7 +100,9 @@ where
         loop {
             match mem::replace(&mut me.state, State::Draining) {
                 State::Watch(on_drain) => {
-                    match me.watch.rx.poll_ref(cx) {
+                    let mut recv_fut = me.watch.rx.recv_ref().boxed();
+
+                    match recv_fut.poll_unpin(cx) {
                         Poll::Ready(None) => {
                             // Drain has been triggered!
                             on_drain(unsafe { Pin::new_unchecked(&mut me.future) });