Fix data race in lookup_confirmations()

Hold the `HeaderList` read lock during lookups to ensure results are
computed against a stable chain tip. Previously, another thread could
update the `HeaderList` between reading the height and querying the DB,
resulting in inconsistent results.

Also adds a missing timer.

(to be squashed)

Author: shesek

src/new_index/schema.rs

@@ -1069,7 +1069,9 @@ impl ChainQuery {
     }
 
     pub fn lookup_confirmations(&self, txids: BTreeSet<Txid>) -> HashMap<Txid, u32> {
-        lookup_confirmations(&self.store.history_db, self.best_height() as u32, txids)
+        let _timer = self.start_timer("lookup_confirmations");
+        let headers = self.store.indexed_headers.read().unwrap();
+        lookup_confirmations(&self.store.history_db, headers.best_height() as u32, txids)
     }
 
     pub fn get_block_status(&self, hash: &BlockHash) -> BlockStatus {