Fix CoinStake transaction

Only worker that finds solution to POS should add output with fees.
Otherwise, race condition occurs and output with fee is added multiple
times and not in right place, causing consensus to reject block with
bad signature exception.

Sources/Stratis.Bitcoin.Features.Miner/DeStreamPosMinting.cs

@@ -40,7 +40,7 @@ namespace Stratis.Bitcoin.Features.Miner
         {
             base.CoinstakeWorker(context, chainTip, block, minimalAllowedTime, searchInterval);
 
-            if (context.Result.KernelFoundIndex == CoinstakeWorkerResult.KernelNotFound)
+            if (context.Result.KernelFoundIndex != context.Index)
                 return;
 
             Script deStreamAddressKey = new KeyId(new uint160(Encoders.Base58Check