Move LayeredTimeline and related code to separate source file.

The layered_repository.rs file had grown to be very large. Split off
the LayeredTimeline struct and related code to a separate source file to
make it more manageable.

There are plans to move much of the code to track timelines from
tenant_mgr.rs to LayeredRepository. That will make layered_repository.rs
grow again, so now is a good time to split it.

There's a lot more cleanup to do, but this commit intentionally only
moves existing code and avoids doing anything else, for easier review.

pageserver/src/layered_repository/layer_map.rs

@@ -10,9 +10,9 @@
 //! corresponding files are written to disk.
 //!
 
+use crate::layered_repository::inmemory_layer::InMemoryLayer;
 use crate::layered_repository::storage_layer::Layer;
 use crate::layered_repository::storage_layer::{range_eq, range_overlaps};
-use crate::layered_repository::InMemoryLayer;
 use crate::repository::Key;
 use anyhow::Result;
 use lazy_static::lazy_static;

pageserver/src/storage_sync.rs

@@ -176,7 +176,6 @@ use crate::{
     layered_repository::{
         ephemeral_file::is_ephemeral_file,
         metadata::{metadata_path, TimelineMetadata, METADATA_FILE_NAME},
-        LayeredRepository,
     },
     storage_sync::{self, index::RemoteIndex},
     tenant_mgr::attach_downloaded_tenants,
@@ -1257,7 +1256,13 @@ async fn update_local_metadata(
             timeline_id,
         } = sync_id;
         tokio::task::spawn_blocking(move || {
-            LayeredRepository::save_metadata(conf, timeline_id, tenant_id, &cloned_metadata, true)
+            crate::layered_repository::save_metadata(
+                conf,
+                timeline_id,
+                tenant_id,
+                &cloned_metadata,
+                true,
+            )
         })
         .await
         .with_context(|| {