Fix syntax error

[andreparames]

The _REDIS_MASTER_POOL was converted from a class variable to an instance variable, but not all uses in the code where updated. The error in the UI Tests was: RuntimeError: 500 500: AttributeError: type object 'RedisPlugin' has no attribute '_REDIS_MASTER_POOL'

Also added explicit connection to Redis on the backup too.

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[sonarcloud]

Quality Gate failed

Failed conditions
D Maintainability Rating on New Code (required ≥ C)

See analysis details on SonarCloud

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