Google Cloud Datastore key conversion

old keys need a fully qualified app id, e.g. s~datastore-key

new keys don't contain an App Id

0 is equivalent to empty Int Id

This old format works well with GAE-specific client libs:

Go google.golang.org/appengine/datastore
Java com.google.appengine.api.datastore.KeyFactory

This format works well with client libs:

Go cloud.google.com/go/datastore