Do not request the same access token when there is a similar request in-flight

[tanguilp]

When requesting an access token, none can be found with the matching scopes and a refresh token exists for the subject and the issuer, a request is made to the token endpoint for a new one.

If the same token is requested while the request is processing, another request is made, and so on until a token is eventually saved in the backend.

We can end up with many access tokens with same specification (for instance, many tokens for the ["scope1", "scope2"] scopes while one would be sufficient.

Solving this issue probably requires dealing with interesting stateful features of Erlang / Elixir.