FR: Batch reads

[ribrdb]

[REQUIRED] Describe your environment

• Operating System version: macOS 10.13.6
• Browser version: Chrome 67
• Firebase SDK version: 5.2.0
• Firebase Product: firestore

There should be an api to fetch a list of document references, like https://firebase.google.com/docs/firestore/reference/rpc/google.firestore.v1beta1#google.firestore.v1beta1.Firestore.BatchGetDocuments

[mikelehen]

Thanks for the suggestion! This is being tracked as #1176