Fix performances obtained with Github auth in tutorial

[leplatrem]

A single PUT on the default bucket generates 10 calls to authentication.unauthenticated_userid().

With basic auth it is hard to notice. But when following the Github auth tutorial, it becomes very obvious that something is wrong with the current proposition :)

I started this PR also mozilla-services/cliquet#641

Note:
Used this trick to track:

    import traceback
    for line in traceback.format_stack():
       print(line.strip())