async setHighestSyncBlock(dbName: string, blockNumber: number, syncing: boolean, rollback = false): Promise<PouchDB.UpsertResponse> {
    // NOTE: dbName, in this case, is actually the id of the record in the SyncStatus db.
    return this.db.upsert(dbName, (document: SyncDocument) => {
      // make sure the truly highest block is always being used
      document.blockNumber = rollback ? blockNumber : Math.max(blockNumber, document.blockNumber);
      document.syncing = syncing;

      // db.upsert sets _rev and _id so we don't have to
      return document;
    });
  }