Support memory-only firestore build

[swftvsn]

Support was added in 7.13.0 (https://firebase.google.com/support/release-notes/js#cloud-firestore_1) of Firebase JavaScript SDK. The import should be in that case

import 'firebase/firestore/memory'; instead of
import 'firebase/firestore';

Not sure what is the best approach to support conditional imports? Anyhow, this is something we'd like to have, as we prefer (due to large amounts of data) to skip the persistence totally.

[jamesdaniels]

My plan is to support it in AngularFire 7.1 in a lazy-loaded version of AngularFirestore.

[swftvsn]

Awesome! This is looking more and more great every release :D I'll take it that you mean 6.1 though?