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.

tests/coordinator/src/tests/batch.rs

@@ -165,7 +165,7 @@ pub async fn batch(
   let signature = Signature(
     schnorrkel::keys::Keypair::from_bytes(&schnorrkel_key_pair)
       .unwrap()
-      .sign_simple(b"substrate", &batch_message(batch.id == 0, &batch))
+      .sign_simple(b"substrate", &batch_message(&batch))
       .to_bytes(),
   );
 

tests/processor/src/tests/batch.rs

@@ -137,10 +137,8 @@ pub(crate) async fn sign_batch(
           messages::substrate::ProcessorMessage::SignedBatch { batch: this_batch },
         ) => {
           if batch.is_none() {
-            assert!(PublicKey::from_raw(key).verify(
-              &batch_message(this_batch.batch.id == 0, &this_batch.batch),
-              &this_batch.signature
-            ));
+            assert!(PublicKey::from_raw(key)
+              .verify(&batch_message(&this_batch.batch), &this_batch.signature));
 
             batch = Some(this_batch.clone());
           }