EventIndex: Cancel the crawler early after a message request.

If we're cancelling the crawler nowadays this means that we're likely
deleting the index. Processing these messages is wasted effort in that
case so break early.

src/indexing/EventIndex.js

@@ -266,6 +266,11 @@ export default class EventIndex extends EventEmitter {
                 continue;
             }
 
+            if (cancelled) {
+                this.crawlerCheckpoints.push(checkpoint);
+                break;
+            }
+
             if (res.chunk.length === 0) {
                 console.log("EventIndex: Done with the checkpoint", checkpoint);
                 // We got to the start/end of our timeline, lets just