Update LoadBalancer.cs

Possible bug if the LoadBalancer lives to process more than `int.MaxValue` messages.
`Interlocked.Increment(ref roundRobinCounter)` would 'overflow' to `int.MinValue` (a negative number) and would cause the `indexOfThisDestination` be negative too, causing an `IndexOutOfRangeException`

>Looked at adding a unit test for it, but would probably run too slow.