Fix OAuth2 authentication in cluster

After calling /oauth2/authorization, the AuthorizationRequest are stored in AuthorizationRequest#requests which is a static Map.

When calling /oauth2/token, we retrieve the AuthorizationRequest from the Map given the authoriation code.
=> If the call /oauth2/token is done on another node, the AuthorizationRequest does not exist.

AuthorizationRequest should probably be stored in Redis to avoid any issue.