Don't upload index file in compaction, if there was nothing to do. (#3149)

This splits the storage_sync2::schedule_index_file into two (public)
functions:
1. `schedule_index_upload_for_metadata_update`, for when the metadata
(e.g. disk_consistent_lsn or last_gc_cutoff) has changed, and

2. `schedule_index_upload_for_file_changes`, for when layer file uploads
or deletions have been scheduled.

We now keep track of whether there have been any uploads or deletions
since the last index-file upload, and skip the upload in
`schedule_index_upload_for_file_changes` if there haven't been any
changes. That allows us to call the function liberally in timeline.rs,
whenever layer file uploads or deletions might've been scheduled,
without starting a lot of unnecessary index file uploads.

GC was covered earlier by commit c262390214, but that missed that we
have the same problem with compaction.

test_runner/regress/test_gc_aggressive.py

@@ -165,6 +165,11 @@ def test_gc_index_upload(neon_env_builder: NeonEnvBuilder, remote_storage_kind:
         cur.execute("INSERT INTO foo VALUES (0, 0, 'foo')")
         pageserver_http.timeline_gc(tenant_id, timeline_id, 10000 - i * 32)
         num_index_uploads = get_num_remote_ops("index", "upload")
+
+        # Also make sure that a no-op compaction doesn't upload the index
+        # file unnecessarily.
+        pageserver_http.timeline_compact(tenant_id, timeline_id)
+
         log.info(f"{num_index_uploads} index uploads after GC iteration {i}")
 
     after = num_index_uploads