pageserver: more reactive wal receiver cancellation (#12076)

## Problem

If the wal receiver is cancelled, there's a 50% chance that it will
ingest yet more WAL.

## Summary of Changes

Always check cancellation first.

pageserver/src/tenant/timeline/walreceiver.rs

@@ -113,7 +113,7 @@ impl WalReceiver {
                 }
                 connection_manager_state.shutdown().await;
                 *loop_status.write().unwrap() = None;
-                debug!("task exits");
+                info!("task exits");
             }
             .instrument(info_span!(parent: None, "wal_connection_manager", tenant_id = %tenant_shard_id.tenant_id, shard_id = %tenant_shard_id.shard_slug(), timeline_id = %timeline_id))
         });

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

@@ -297,6 +297,7 @@ pub(super) async fn handle_walreceiver_connection(
     let mut expected_wal_start = startpoint;
     while let Some(replication_message) = {
         select! {
+            biased;
             _ = cancellation.cancelled() => {
                 debug!("walreceiver interrupted");
                 None