feat: Logical termination for firestore.getAll(...).

[ehsannas]

No description provided.

[ehsannas]

This improves the performance for 2 cases:

(1)

firestore.getAll(docRef1, docRef2, docRef3).get()

we know we expect 3 responses, so we can return the result after we receive 3 docs.

(2)

docRef.get().get()

This simply calls firestore.getAll(docRef) under the hood. We know we expect 1 document result, so we can return the result as soon as we receive 1 document

(b/309863147)

[cherylEnkidu]

Hi Ehsan, I change the unit tests to verify the changes. Please only merge the PR If my changes look good to you,

[cherylEnkidu]

Hi @ehsannas , please verify my latest fix before merging the PR. Thank you!