Reduce mgets executed by backend-worker to avoid redundant checks for usage alerts

pull/221 was a performance optimization that reduced the load on Redis by reducing the keys queried by backend-listener to the minimum necessary.

pull/275 brings some further improvement on this area by reducing the amount of keys read from Redis.

This request is to investigate possible ways to further reduce the amount of mgets executed by backend-worker to avoid redundant checks, depending on what usage alerts are configured.