optimize repeated reloads of the flow classification engine

While editing flow classification rules, repeated reloads of the flow classification engine are triggered. Interactive trial classifications are blocked until ongoing reloads are finished. Blocking can take at most 2 times the time it takes to reload the classification engine because while a reload is in process another "pending" reload may have been registered.

The behavior can be improved if in case of a necessary reload any ongoing reload is cancelled and the required reload starts immediately.