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(),
   );