[PULP-501] Create a parallel v4 API

[dralley]

closes #6462

[dralley]

TODO:

• requesting v4 APIs doesn't seem to work for most of them, e.g. /v4/status/ works but /v4/tasks/ doesn't
• reversing namespaced endpoint names is more complicated and probably broken in a number of locations
• ensure that adding names to endpoints which previously did not have names specified does not break the bindings

[mdellweg]

pop(..., None) would work too, and I would call it resilient, not hacky. ;)