Correct the prior documented TOCTOU

Now, if a malicious validator set publishes a malicious `Batch` at the last
moment, it'll cause all future `Batch`s signed by the next validator set to
require a bool being set (yet they never will set it).

This will prevent the handover.

The only overhead is having two distinct `batch_message` calls on-chain.

coordinator/src/main.rs

@@ -924,15 +924,6 @@ async fn handle_processor_messages<D: Db, Pro: Processors, P: P2p>(
                   MainDb::<D>::set_did_handover(&mut txn, spec.set());
                 }
 
-                // TODO: There is a race condition here. We may verify all `Batch`s from the prior
-                // set, start signing the handover Batch `n`, start signing `n+1`, have `n+1`
-                // signed before `n` (or at the same time), yet then the prior set forges a
-                // malicious Batch `n`.
-                //
-                // The malicious Batch `n` would be publishable to Serai, as Serai can't
-                // distinguish what's intended to be a handover `Batch`, yet then anyone could
-                // publish the new set's `n+1`, causing their acceptance of the handover.
-
                 Some(Transaction::Batch(block.0, id.id))
               } else {
                 Some(Transaction::BatchPreprocess(SignData {