Update y-indexeddb.js

start update after sync to avoid yjs#25 and yjs#31 and listen for updates after the sync to avoid duplicating updates.

Client will need to ensure no updates are made before sync is completed. A more robust implementation would need to queue updates.

src/y-indexeddb.js

@@ -120,7 +120,7 @@ export class IndexeddbPersistence extends Observable {
         }
       }
     }
-    doc.on('update', this._storeUpdate)
+    this.whenSynced.then(() => doc.on('update', this._storeUpdate))
     this.destroy = this.destroy.bind(this)
     doc.on('destroy', this.destroy)
   }