pageserver: un-Arc Timeline::layers (#8386)

## Problem

This structure was in an Arc<> unnecessarily, making it harder to reason
about its lifetime (i.e. it was superficially possible for LayerManager
to outlive timeline, even though no code used it that way)

## Summary of changes

- Remove the Arc<>

pageserver/src/tenant/timeline.rs

@@ -198,7 +198,7 @@ impl PartialOrd for Hole {
 
 /// Temporary function for immutable storage state refactor, ensures we are dropping mutex guard instead of other things.
 /// Can be removed after all refactors are done.
-fn drop_rlock<T>(rlock: tokio::sync::OwnedRwLockReadGuard<T>) {
+fn drop_rlock<T>(rlock: tokio::sync::RwLockReadGuard<T>) {
     drop(rlock)
 }
 
@@ -271,7 +271,7 @@ pub struct Timeline {
     ///
     /// In the future, we'll be able to split up the tuple of LayerMap and `LayerFileManager`,
     /// so that e.g. on-demand-download/eviction, and layer spreading, can operate just on `LayerFileManager`.
-    pub(crate) layers: Arc<tokio::sync::RwLock<LayerManager>>,
+    pub(crate) layers: tokio::sync::RwLock<LayerManager>,
 
     last_freeze_at: AtomicLsn,
     // Atomic would be more appropriate here.

pageserver/src/tenant/timeline/compaction.rs

@@ -27,8 +27,8 @@ use utils::id::TimelineId;
 use crate::context::{AccessStatsBehavior, RequestContext, RequestContextBuilder};
 use crate::page_cache;
 use crate::tenant::storage_layer::{AsLayerDesc, PersistentLayerDesc};
-use crate::tenant::timeline::{drop_rlock, Hole, ImageLayerCreationOutcome};
-use crate::tenant::timeline::{DeltaLayerWriter, ImageLayerWriter};
+use crate::tenant::timeline::{drop_rlock, DeltaLayerWriter, ImageLayerWriter};
+use crate::tenant::timeline::{Hole, ImageLayerCreationOutcome};
 use crate::tenant::timeline::{Layer, ResidentLayer};
 use crate::tenant::DeltaLayer;
 use crate::virtual_file::{MaybeFatalIo, VirtualFile};
@@ -379,7 +379,7 @@ impl Timeline {
             };
 
             let begin = tokio::time::Instant::now();
-            let phase1_layers_locked = Arc::clone(&self.layers).read_owned().await;
+            let phase1_layers_locked = self.layers.read().await;
             let now = tokio::time::Instant::now();
             stats.read_lock_acquisition_micros =
                 DurationRecorder::Recorded(RecordedDuration(now - begin), now);
@@ -399,9 +399,9 @@ impl Timeline {
     }
 
     /// Level0 files first phase of compaction, explained in the [`Self::compact_legacy`] comment.
-    async fn compact_level0_phase1(
-        self: &Arc<Self>,
-        guard: tokio::sync::OwnedRwLockReadGuard<LayerManager>,
+    async fn compact_level0_phase1<'a>(
+        self: &'a Arc<Self>,
+        guard: tokio::sync::RwLockReadGuard<'a, LayerManager>,
         mut stats: CompactLevel0Phase1StatsBuilder,
         target_file_size: u64,
         ctx: &RequestContext,