Minimize network roundtrip in PersonalAccessToken#redis_getdel and make it more reliable

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As mentioned in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22916#note_117630911:

We use a Redis pipeline to do the GET and DELETE request so we don't have to make 2 network roundtrips. Redis pipelines aren't atomic, so we may need MULTI/EXEC.

There's a small possibility that if an admin creates two tokens close together, he/she sees the wrong one.

/cc @stanhu