Verify all Batchs published by the prior set

The new set publishing a `Batch` completes the handover protocol. The new set
should only publish a `Batch` once it believes the old set has completed all of
its on-external-chain activity, marking it honest and finite.

With the handover comes the acceptance of liability, hence the requirement for
all of the on-Serai-chain activity also needing verification. While most
activity would be verified in-real-time (upon ::Batch messages), the new set
will now explicitly verify the complete set of `Batch`s before beginning its
preprocess for its own `Batch` (the one accepting the handover).

substrate/client/src/serai/validator_sets.rs

@@ -61,7 +61,9 @@ impl Serai {
     key: Public,
     at_hash: [u8; 32],
   ) -> Result<Option<Amount>, SeraiError> {
-    self.storage(PALLET, "Allocations", Some(vec![scale_value(network), scale_value(key)]), at_hash).await
+    self
+      .storage(PALLET, "Allocations", Some(vec![scale_value(network), scale_value(key)]), at_hash)
+      .await
   }
 
   pub async fn get_validator_set_musig_key(