code: function() {
          return Promise.try(() => {
              const txn = this.db.transaction(["users"], "readonly");
              const store = txn.objectStore("users");
              return selectQuery(store, undefined, (cursor) => {
                  return [cursor.value.userId, cursor.value.event];
              });
          });
      }
docs: Load all user presence events from the database. This is not cached.
  FIXME: It would probably be more sensible to store the events in the
  sync.
  @return {Promise<Object[]>} A list of presence events in their raw form.