refactor: single phase Timeline::load_layer_map (#5074)

Current implementation first calls `load_layer_map`, which loads all
local layers, cleans up files, leave cleaning up stuff to "second
function". Then the "second function" is finally called, it does not do
the cleanup and some of the first functions setup can torn down. "Second
function" is actually both `reconcile_with_remote` and
`create_remote_layers`.

This change makes it a bit more verbose but in one phase with the
following sub-steps:
1. scan the timeline directory
2. delete extra files
    - now including on-demand download files
    - fixes #3660
3. recoincile the two sources of layers (directory, index_part)
4. rename_to_backup future layers, short layers
5. create the remaining as layers

Needed by #4938.

It was also noticed that this is blocking code in an `async fn` so just
do it in a `spawn_blocking`, which should be healthy for our startup
times. Other effects includes hopefully halving of `stat` calls; extra
calls which were not done previously are now done for the future layers.

Co-authored-by: Christian Schwarz <christian@neon.tech>
Co-authored-by: John Spray <john@neon.tech>

test_runner/regress/test_ondemand_download.py

@@ -369,7 +369,7 @@ def test_download_remote_layers_api(
     filled_current_physical = get_api_current_physical_size()
     log.info(filled_current_physical)
     filled_size = get_resident_physical_size()
-    log.info(filled_size)
+    log.info(f"filled_size: {filled_size}")
     assert filled_current_physical == filled_size, "we don't yet do layer eviction"
 
     env.pageserver.stop()
@@ -377,7 +377,7 @@ def test_download_remote_layers_api(
     # remove all the layer files
     # XXX only delete some of the layer files, to show that it really just downloads all the layers
     for layer in (Path(env.repo_dir) / "tenants").glob("*/timelines/*/*-*_*"):
-        log.info(f"unlinking layer {layer}")
+        log.info(f"unlinking layer {layer.name}")
         layer.unlink()
 
     # Shut down safekeepers before starting the pageserver.
@@ -403,7 +403,7 @@ def test_download_remote_layers_api(
         filled_current_physical == get_api_current_physical_size()
     ), "current_physical_size is sum of loaded layer sizes, independent of whether local or remote"
     post_unlink_size = get_resident_physical_size()
-    log.info(post_unlink_size)
+    log.info(f"post_unlink_size: {post_unlink_size}")
     assert (
         post_unlink_size < filled_size
     ), "we just deleted layers and didn't cause anything to re-download them yet"