Also moving out the response parsing into a helper which can
be monkey-patched out during testing.

This is for the `v1beta2` to `v1beta3` change. Currently the
`CommitResponse` object contains a single `MutationResult` that holds
an integer `index_updates` field and a repeated field (`insert_auto_id_key`)
of partial `Key`s that were completed in the `commit`.

In `v1beta3`, the `CommitResponse` directly holds the integer `index_updates`
as well as a list of `MutationResult`s. Additionally, the definition of
`MutationResult` no longer has `index_updates` (obviously given the last
sentence) and no longer holds a list of `Key`s. Instead, it holds a single
`Key` in the `key` field. What's more, the list of `MutationResult`s
corresponds to every single `Entity` modified in the commit. For those
where a partial `Key` was completed, the `MutationResult.key` holds the
new key. For those were **no key was completed**, `MutationResult.key`
is empty.