Adding mutex to functions that use ForEachMaster method

[mativm02]

Description

ForEachMaster is a Redis' method that concurrently iterates over each master of the cluster instance. We weren't using a mutex, so this caused a race condition and the final results were randomly incorrect.
Also, I deleted the return of cursor in GetKeysWithOpts since we're not using it nowhere and it causes confusion when the instance is a cluster (since each master node could return a different cursor).

New GetKeysWithOpts can be used in the following way:

var cursor map[string]uint64
var keys []string
var continueScan bool
var err error
for {
  keys, newCursor, continueScan, err = kv.GetKeysWithOpts(ctx, "keys*", continueScan, 10)
  if err != nil {
    return err
  }

 // handle logic

 if !continueScan {
  break
}
}

Note that it's important to use the returned cursor map on each new GetKeysWithOpts call.

Related Issue

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Test Coverage For This Change

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Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactoring or add test (improvements in base code or adds test coverage to functionality)
• Documentation updates or improvements.

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[sonarqubecloud]

Quality Gate passed

The SonarCloud Quality Gate passed, but some issues were introduced.

3 New issues
0 Security Hotspots
87.9% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud