Cache the block's events within TemporalSerai

Event retrieval was prior:
- Retrieve all events in the block, which may be hundreds of KB
- Filter to just a few

Since it's frequent to want multiple sets of events, each filtered in their own
way, this caused the retrieval to happen multiple times. Now, it only will
happen once.

Also has the scoped clients take a reference, not an owned TemporalSerai.

coordinator/src/substrate/mod.rs

@@ -62,7 +62,8 @@ async fn handle_new_set<D: Db>(
     log::info!("present in set {:?}", set);
 
     let set_data = {
-      let serai = serai.as_of(block.hash()).validator_sets();
+      let serai = serai.as_of(block.hash());
+      let serai = serai.validator_sets();
       let set_participants =
         serai.participants(set.network).await?.expect("NewSet for set which doesn't exist");