pagekeeper: suppress error logs in shutdown/detach (#4876)

## Problem

Error messages like this coming up during normal operations:
```
        Compaction failed, retrying in 2s: timeline is Stopping

       Compaction failed, retrying in 2s: Cannot run compaction iteration on inactive tenant
```

## Summary of changes

Add explicit handling for the shutdown case in these locations, to
suppress error logs.

pageserver/src/tenant/tasks.rs

@@ -103,6 +103,11 @@ async fn compaction_loop(tenant: Arc<Tenant>, cancel: CancellationToken) {
                 }
             }
 
+            if cancel.is_cancelled() {
+                info!("received cancellation request");
+                break;
+            }
+
             let started_at = Instant::now();
 
             let sleep_duration = if period == Duration::ZERO {

pageserver/src/tenant/timeline.rs

@@ -697,6 +697,9 @@ impl Timeline {
                 Err(CompactionError::DownloadRequired(rls)) => {
                     anyhow::bail!("Compaction requires downloading multiple times (last was {} layers), possibly battling against eviction", rls.len())
                 }
+                Err(CompactionError::ShuttingDown) => {
+                    return Ok(());
+                }
                 Err(CompactionError::Other(e)) => {
                     return Err(e);
                 }
@@ -778,7 +781,8 @@ impl Timeline {
         let layer_removal_cs = Arc::new(self.layer_removal_cs.clone().lock_owned().await);
         // Is the timeline being deleted?
         if self.is_stopping() {
-            return Err(anyhow::anyhow!("timeline is Stopping").into());
+            trace!("Dropping out of compaction on timeline shutdown");
+            return Err(CompactionError::ShuttingDown);
         }
 
         let target_file_size = self.get_checkpoint_distance();
@@ -3235,6 +3239,8 @@ enum CompactionError {
     /// This should not happen repeatedly, but will be retried once by top-level
     /// `Timeline::compact`.
     DownloadRequired(Vec<Arc<RemoteLayer>>),
+    /// The timeline or pageserver is shutting down
+    ShuttingDown,
     /// Compaction cannot be done right now; page reconstruction and so on.
     Other(anyhow::Error),
 }