Implement allocate_ids for datastore

[dhermes]

I discovered this when checking out the gcloud-node regression tests.

[tseaver]

I'm not sure what that test in the node code is exercising, but it seems like it might be the following:

https://github.com/GoogleCloudPlatform/gcloud-python/blob/41f97e187173e9f36930258bfd9e54048d22fe97/gcloud/datastore/transaction.py#L237-L240

where the transaction updates partial keys after committing, using the results from the commit response.

[silvolu]

@tseaver is this endpoint of the API

[tseaver]

I found the protobuf request and response