Continued startup speedup (#4372)

Startup continues to be slow, work towards to alleviate it.

Summary of changes:

- pretty the functional improvements from #4366 into
`utils::completion::{Completion, Barrier}`
- extend "initial load completion" usage up to tenant background tasks
    - previously only global background tasks
- spawn_blocking the tenant load directory traversal
- demote some logging
- remove some unwraps
- propagate some spans to `spawn_blocking`

Runtime effects should be major speedup to loading, but after that, the
`BACKGROUND_RUNTIME` will be blocked for a long time (minutes). Possible
follow-ups:
- complete initial tenant sizes before allowing background tasks to
block the `BACKGROUND_RUNTIME`

test_runner/regress/test_tenants.py

@@ -309,7 +309,7 @@ def test_pageserver_with_empty_tenants(
     env.pageserver.allowed_errors.append(
         ".*marking .* as locally complete, while it doesnt exist in remote index.*"
     )
-    env.pageserver.allowed_errors.append(".*load failed.*Failed to list timelines directory.*")
+    env.pageserver.allowed_errors.append(".*load failed.*list timelines directory.*")
 
     client = env.pageserver.http_client()