code: function() {
          console.log(
              `LocalIndexedDBStoreBackend: loading sync data...`,
          );
          return Promise.try(() => {
              const txn = this.db.transaction(["sync"], "readonly");
              const store = txn.objectStore("sync");
              return selectQuery(store, undefined, (cursor) => {
                  return cursor.value;
              }).then((results) => {
                  console.log(
                      `LocalIndexedDBStoreBackend: loaded sync data`,
                  );
                  if (results.length > 1) {
                      console.warn("loadSyncData: More than 1 sync row found.");
                  }
                  return (results.length > 0 ? results[0] : {});
              });
          });
      }
docs: Load the sync data from the database.
  @return {Promise<Object>} An object with "roomsData" and "nextBatch" keys.