async updateServer_() {
    if (this.writeInProgress || this.notSent.length == 0) {
      return;
    }
    this.writeInProgress = true;
    while (this.notSent.length > 0) {
      await this.writeToDatabase_();
    }
    this.writeInProgress = false;
  }
Send local changes to the server. Continuously runs until all local changes
have been sent.
@private