Repair hashrate capture in ethhashmining coordinator (#39)

On the transition to the EthHash/Clique Mining Coordinator a change
was lost whereby the EthHashMiningCoordinator is to capture the
current Hashrate on cancellation of a mining operation.

ethereum/core/src/main/java/net/consensys/pantheon/ethereum/blockcreation/EthHashMiningCoordinator.java

@@ -58,4 +58,13 @@ public class EthHashMiningCoordinator extends AbstractMiningCoordinator<Void, Et
       return currentRunningMiner.map(miner -> miner.submitWork(solution)).orElse(false);
     }
   }
+
+  @Override
+  protected void haltCurrentMiningOperation() {
+    currentRunningMiner.ifPresent(
+        miner -> {
+          miner.cancel();
+          miner.getHashesPerSecond().ifPresent(val -> cachedHashesPerSecond = Optional.of(val));
+        });
+  }
 }