* Fixes the concurrency issue of calling the `Next()` proxy target on round robin balancer
- fixed concurrency issue in `AddTarget()`
- moved `rand.New()` to the random balancer initializer func.
- internal code reorganized eliminating unnecessary pointer redirection
- employing `sync.Mutex` instead of `RWMutex` which brings additional overhead of tracking readers and writers. No need for that since the guarded code has no long-running operations, hence no realistic congestion.
- added additional guards without which the code would otherwise panic (e.g., the case where a random value is calculation when targets list is empty)
- added descriptions for func return values, what to expect in which case.
- Improve code test coverage
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Co-authored-by: Becir Basic <bb@neotel.at>
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