core: reset DKG (#502)

* Allow utils.NodeSetCache to purge by rounds.

* Purge utils.NodeSetCache when DKG reset.

* Add a utils.RoundEvent handler to abort all
  previous running DKG

* Fix test.App hangs in BlockDelivered when
  utils.RoundEvent is attached.

  ValidateNextRound is a blocking call and would
  block test.App.BlockDelivered.

1 files changed, 9 insertions, 0 deletions

diff --git a/core/utils/crypto.go b/core/utils/crypto.go
index 7532d29..8be503f 100644
--- a/core/utils/crypto.go
+++ b/core/utils/crypto.go
@@ -323,3 +323,12 @@ func VerifyDKGFinalizeSignature(
 	}
 	return true, nil
 }
+
+// Rehash hashes the hash again and again and again...
+func Rehash(hash common.Hash, count uint) common.Hash {
+	result := hash
+	for i := uint(0); i < count; i++ {
+		result = crypto.Keccak256Hash(result[:])
+	}
+	return result
+}