fix(pageserver): allow shard ancestor compaction to be cancelled (#11452)

## Problem

https://github.com/neondatabase/neon/issues/11330
https://github.com/neondatabase/neon/issues/11358

## Summary of changes

Looking at the staging log, a few tenants right after shard split are
stuck on shutdown because they are running shard ancestor compaction.
The compaction does not respect the cancellation token.

Signed-off-by: Alex Chi Z <chi@neon.tech>

pageserver/src/tenant/timeline/compaction.rs

@@ -1244,6 +1244,10 @@ impl Timeline {
         let mut replace_image_layers = Vec::new();
 
         for layer in layers_to_rewrite {
+            if self.cancel.is_cancelled() {
+                return Err(CompactionError::ShuttingDown);
+            }
+
             tracing::info!(layer=%layer, "Rewriting layer after shard split...");
             let mut image_layer_writer = ImageLayerWriter::new(
                 self.conf,