Add is_stopping check around critical macro in walreceiver (#11496)

The timeline stopping state is set much earlier than the cancellation
token is fired, so by checking for the stopping state, we can prevent
races with timeline shutdown where we issue a cancellation error but the
cancellation token hasn't been fired yet.

Fix #11427.

Author: arpad-m

pageserver/src/tenant/timeline/walreceiver/walreceiver_connection.rs

@@ -445,7 +445,7 @@ pub(super) async fn handle_walreceiver_connection(
                         .inspect_err(|err| {
                             // TODO: we can't differentiate cancellation errors with
                             // anyhow::Error, so just ignore it if we're cancelled.
-                            if !cancellation.is_cancelled() {
+                            if !cancellation.is_cancelled() && !timeline.is_stopping() {
                                 critical!("{err:?}")
                             }
                         })?;
@@ -577,7 +577,7 @@ pub(super) async fn handle_walreceiver_connection(
                             .inspect_err(|err| {
                                 // TODO: we can't differentiate cancellation errors with
                                 // anyhow::Error, so just ignore it if we're cancelled.
-                                if !cancellation.is_cancelled() {
+                                if !cancellation.is_cancelled() && !timeline.is_stopping() {
                                     critical!("{err:?}")
                                 }
                             })?;