oauth2: requests waste a lot of time in fosite storer `requestFromRDB()` routine

[aeneasr]

Reported by @faxal here: https://github.com/ory-am/hydra/blob/3c3b93e6886a473c9472fdb61c584acda3d758aa/internal/fosite_store_rethinkdb.go#L50

The solution would be to keep the result in memory.

[aeneasr]

RethinkDB will no longer be actively maintained without customer requests and is superseded by #292