incorporate CachedOAuth2Validator improvement from LuizaLabs

[n2ygk]

Is your feature request related to a problem? Please describe.
In production operation with DRF/DJA using OAuth2Validator to validate Bearer Access Tokens (provided by an external AS) we've seen excessive SQL database lookups (one per API endpoint invocation) that are contributing unnecessary load to our database server. A typical user's Access Token can result in hundreds of closely-spaced REST API calls and the corresponding database AccessToken model queries.

Describe the solution you'd like
LuizaLabs has fixed this to use a cache as described here:

https://django-toolkit.readthedocs.io/en/stable/oauth2/

and implemented here:

https://github.com/luizalabs/django-toolkit/blob/master/django_toolkit/oauth2/validators.py

[n2ygk]

Investigate whether caching relevant parts of the UserModel would be needed as well.
select_related already includes the full UserModel instance?