DKG improvements to the challenge mechanism

[pdyraga]

There are two problems with the existing challenge mechanism:

1. In case we run the challenge loop in executeDkgValidation one more time, we do not know if the result is still the one most recently submitted to the chain. It could happen that the challenge was mined and another result got submitted later. Before we execute this loop again, we should check the result - probably reading it from the last event.
2. The log saying invalid DKG result still not challenged; retrying may not always be true. We could have another result submitted in the meantime and challenged. I think we should remove this log from the end of the loop and move it to the beginning of the loop after we check that the result we are challenging is the last one submitted.