Backend multitenancy
See https://cloud.google.com/appengine/docs/standard/go/multitenancy/multitenancy
Any alternative "version" of the backend, even assigned 0% traffic, poses a risk of confusion of the Datastore data, the Memcache data, the Search indexes, and the delayed Tasks.
Make sure that alternative versions can never mess up with the prod data.
This may be taken care of in funcs handle and handleAjax, with (*Request).WithContext and appengine.Namespace
Not that easy: I end up with panic: appengine: NewContext passed an unknown http.Request
https://github.com/Deleplace/programming-idioms/commits/issues/86-namespaces