cometindex: avoid unnecessary rollbacks (#5106)

We created a transaction too early in the previous version of the code,
which meant that in the common case of skipping a batch of transactions,
we would create the transaction before checking if we need to do
anything.

Smoke tests should be sufficient to test.

## Checklist before requesting a review

- [x] I have added guiding text to explain how a reviewer should test
these changes.

- [x] If this code contains consensus-breaking changes, I have added the
"consensus-breaking" label. Otherwise, I declare my belief that there
are not consensus-breaking changes, for the following reason:

  > indexing only

crates/util/cometindex/src/indexer.rs

@@ -55,7 +55,6 @@ async fn catchup(
                 tracing::info!(?name, "already initialized");
             }
             while let Some(mut events) = rx.recv().await {
-                let mut dbtx = state_cp.begin_transaction().await?;
                 // We only ever want to index events past our current height.
                 // We might receive a batch with more events because other indices are behind us.
                 events.start_later(index_height.next().into());
@@ -75,6 +74,7 @@ async fn catchup(
                     "indexing batch"
                 );
                 let last_height = events.last_height();
+                let mut dbtx = state_cp.begin_transaction().await?;
                 index.index_batch(&mut dbtx, events).await?;
                 tracing::debug!(index_name = &name, "committing batch");
                 IndexingState::update_index_height(&mut dbtx, &name, Height::from(last_height))