pageserver: refined Timeline shutdown (#5833)

## Problem

We have observed the shutdown of a timeline taking a long time when a
deletion arrives at a busy time for the system. This suggests that we
are not respecting cancellation tokens promptly enough.

## Summary of changes

- Refactor timeline shutdown so that rather than having a shutdown()
function that takes a flag for optionally flushing, there are two
distinct functions, one for graceful flushing shutdown, and another that
does the "normal" shutdown where we're just setting a cancellation token
and then tearing down as fast as we can. This makes things a bit easier
to reason about, and enables us to remove the hand-written variant of
shutdown that was maintained in `delete.rs`
- Layer flush task checks cancellation token more carefully
- Logical size calculation's handling of cancellation tokens is
simplified: rather than passing one in, it respects the Timeline's
cancellation token.

This PR doesn't touch RemoteTimelineClient, which will be a key thing to
fix as well, so that a slow remote storage op doesn't hold up shutdown.

pageserver/src/http/routes.rs

@@ -303,11 +303,7 @@ async fn build_timeline_info(
         // we're executing this function, we will outlive the timeline on-disk state.
         info.current_logical_size_non_incremental = Some(
             timeline
-                .get_current_logical_size_non_incremental(
-                    info.last_record_lsn,
-                    CancellationToken::new(),
-                    ctx,
-                )
+                .get_current_logical_size_non_incremental(info.last_record_lsn, ctx)
                 .await?,
         );
     }