dev: make PreferLocations function pure

[pelageech]

balancers.PreferLocations should be pure function and not modify the slice.

func PreferLocations(balancer *balancerConfig.Config, locations ...string) *balancerConfig.Config {
    if len(locations) == 0 {
        panic("empty list of locations")
    }
    for i := range locations { // need to copy the slice
        locations[i] = strings.ToUpper(locations[i])
    }
    sort.Strings(locations)
    balancer.Filter = filterLocations(locations)

    return balancer
}