Implement one-time get for Firestore and Realtime Database

[jhuleatt]

The Firebase JS SDK has a get method for Firestore and Realtime Database that fetches a value once and doesn't stream updates.

We should probably implement something like a useFirestoreDocOnce to allow for this behavior in reactfire. Or, add a once flag to ReactFireOptions to accomplish the same thing.

• useFirestoreDoc[Data]Once Add hooks to get firestore document data once #211
• useFirestoreCollection[Data]Once
• useDatabaseObject[Data]Once
• useDatabaseList[Data]Once

[forrest-keller]

I think we should use getFirestoreDoc, as used in the official Firebase APK