Add an index on AccessToken.token to speedup token verification

[tchaumeny]

A report on our PostgreSQL database use showed that the most consuming query was this one :

SELECT "oauth2_provider_accesstoken"."id", "oauth2_provider_accesstoken"."user_id", "oauth2_provider_accesstoken"."token", "oauth2_provider_accesstoken"."application_id", "oauth2_provider_accesstoken"."expires", "oauth2_provider_accesstoken"."scope" FROM "oauth2_provider_accesstoken" WHERE "oauth2_provider_accesstoken"."token" = '';

This pull request adds an index on the AccessToken.token field so that this very frequent query can run much faster.

[coveralls]

Coverage remained the same when pulling f5da293 on Polyconseil:speedup_token into e983442 on evonove:master.

[synasius]

that would be nice to merge but it's breaking tests.. please try to fix that or we can't merge!

[tchaumeny]

Hi @synasius. This failure is not related to my commit and happens randomly on some build (see https://travis-ci.org/evonove/django-oauth-toolkit/jobs/29046466 for instance).

[palazzem]

Hi @tchaumeny,
yes I can confirm that sometimes there are some failures related to Travis environment. I've launched the job again and now it says that everything is fine (at least everything which is covered by tests :)).
I leave to @synasius the last code review and the merge operation.

Thanks for your PR!

[tchaumeny]

In fact, this was already done in d56ad55.

[n2ygk]

I'm guessing that if indexeing was manually added to a migration back a long time that it got reverted if it's not in the model definition proper. I see the #1312 addresses this by adding db_index = True in the model definition.