Link to the issue for downloading transactions

Make it easy to navigate from the `TODO` comment to the current project
planning.

Co-authored-by: teor <teor@riseup.net>

zebrad/src/components/mempool/crawler.rs

@@ -92,7 +92,7 @@ where
             transaction_ids.len()
         );
 
-        // TODO: Download transactions and send them to the mempool
+        // TODO: Download transactions and send them to the mempool (#2650)
 
         Ok(())
     }