refactor(timeline_detach_ancestor): replace ordered reparented with a hashset (#8629)

Earlier I was thinking we'd need a (ancestor_lsn, timeline_id) ordered
list of reparented. Turns out we did not need it at all. Replace it with
an unordered hashset. Additionally refactor the reparented direct
children query out, it will later be used from more places.

Split off from #8430.

Cc: #6994

storage_controller/src/service.rs

@@ -2954,7 +2954,6 @@ impl Service {
         }
 
         // no shard needs to go first/last; the operation should be idempotent
-        // TODO: it would be great to ensure that all shards return the same error
         let mut results = self
             .tenant_for_shards(targets, |tenant_shard_id, node| {
                 futures::FutureExt::boxed(detach_one(
@@ -2973,6 +2972,7 @@ impl Service {
             .filter(|(_, res)| res != &any.1)
             .collect::<Vec<_>>();
         if !mismatching.is_empty() {
+            // this can be hit by races which should not happen because operation lock on cplane
             let matching = results.len() - mismatching.len();
             tracing::error!(
                 matching,