Get rid of Class variable storage and move to instance storage

I'm building an admin/monitoring server that handles requests from both out production and qa Shopify store.
In the current setup, the user/password/site_uri are stored at the class level, so I can't create a prod session and a qa session and store them in memory.
For every request to the REST server I have to clear the current class variables and set for requested environment.

Additionally, this is not thread safe. If two REST requests arrive at the same time for different endpoints, they will clobber each other.

Empieza rastreando dónde se almacenan user, password y site_uri y cómo se accede a ellos para las solicitudes REST; después, revisa cómo se crean y seleccionan las sesiones para distintos entornos de Shopify. Se considera terminado cuando las sesiones de producción y QA pueden coexistir en memoria y las solicitudes concurrentes ya no sobrescriben la configuración de las demás; añade cobertura para ambos casos o ejecútala.