async documentIsProcessed(filename) {
    const value = await this.docsClient.get(filename);
    if (value) {
      console.log(`${filename} already added to index.`);
      return true;
    }
    if (value === '') {
      console.log(`${filename} was already checked, and contains no text.`);
      return true;
    }
    return false;
  }
Check to see if a Document is already stored in redis.
@param {string} filename
@returns {Promise<boolean>}