firestore client: breaking change

[Jonathan Amsterdam]

We just submitted https://code-review.googlesource.com/c/gocloud/+/31691, which is a breaking change to cloud.google.com/go/firestore.

Before this change, QuerySnapshotIterator.Next returned a DocumentIterator containing the result for the current query snapshot. Other properties of the snapshot, like the list of changes and the result size, were fields of the QuerySnapshotIterator value. This split between the query results and the other properties was confusing.

In this change, QuerySnapshotIterator.Next returns a QuerySnapshot value which holds everything about the snapshot. See this example for usage.

If you work at head with GOPATH and "go get -u", you'll see this change the next time you update. Otherwise, you'll see this change the next time we do a release.