Alternate handover batch TOCTOU fix (#397)

* Revert "Correct the prior documented TOCTOU"

This reverts commit d50fe87801.

* Correct the prior documented TOCTOU

d50fe87801 edited the challenge for the Batch to
fix it. This won't produce Batch n+1 until Batch n is successfully published
and verified. It's an alternative strategy able to be reviewed, with a much
smaller impact to scope.

substrate/client/tests/common/in_instructions.rs

@@ -39,8 +39,7 @@ pub async fn provide_batch(batch: Batch) -> [u8; 32] {
 
   let block = publish_tx(&Serai::execute_batch(SignedBatch {
     batch: batch.clone(),
-    // TODO: This `batch.id == 0` line only works when session == 0
-    signature: pair.sign(&batch_message(batch.id == 0, &batch)),
+    signature: pair.sign(&batch_message(&batch)),
   }))
   .await;