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.

libs/utils/src/seqwait.rs

@@ -125,6 +125,9 @@ where
             // Wake everyone with an error.
             let mut internal = self.internal.lock().unwrap();
 
+            // Block any future waiters from starting
+            internal.shutdown = true;
+
             // This will steal the entire waiters map.
             // When we drop it all waiters will be woken.
             mem::take(&mut internal.waiters)