title: Firestore - Using cache until online content updates
body: <p>I am starting with Firestore. I've read docs and tutorials about the offline data persistence but I have not really clear if Firestore downloads data again even if the content hasn't been modified. 
  For example, if I have a query where the results will be updated once a week and I don't need that the app download the content again until the changes were made, what is the best way in terms of efficiency to write the code? 
  Thanks!</p>