FR: Firestore getAll() to retrieve multiple documents at once.

### [REQUIRED] Describe your environment

* Operating System version: macOS
* Browser version: chrome Version 67.0.3396.99
* Firebase SDK version: 5.4.1
* Firebase Product: firestore

### [REQUIRED] Describe the problem
A feature request for Firestore transaction.getAll
according to documentation transaction.getAll supported by Node.JS
https://cloud.google.com/nodejs/docs/reference/firestore/0.15.x/Transaction#getAll

I though it's available on web sdk but got error.. ( transaction.getAll is not a function )

Already use Promise.all(transaction.get(ref1),transaction.get(ref2),transaction.get(ref3))
and its working fine as alternative solution.

So it not that important FR, but i didn't understand why exist on Node but not on web sdk.

thanks

Start by comparing the Firestore web SDK transaction API with the Node.js Transaction.getAll documentation linked in the issue. Review how transaction.get is exposed and use the reported Promise.all alternative as the behavioral reference; done means web transactions can retrieve multiple document references through getAll without the current “not a function” error.

Written by the indexing model from the issue text.