RPC call gettransactionheight (neo-project#541)

* getrawtransactionheight

Nowadays two calls are need to get a transaction height, `getrawtransaction` with `verbose` and then use the `blockhash`.
Other option is to use `confirmations`, but it can be misleading.

* Minnor fix

* Shargon's tip

* modified

neo/Network/RPC/RpcServer.cs

@@ -331,6 +331,13 @@ private JObject Process(string method, JArray _params)
                         }) ?? new StorageItem();
                         return item.Value?.ToHexString();
                     }
+                case "gettransactionheight":
+                    {
+                        UInt256 hash = UInt256.Parse(_params[0].AsString());
+                        uint? height = Blockchain.Singleton.Store.GetTransactions().TryGet(hash)?.BlockIndex;
+                        if (height.HasValue) return height.Value;
+                        throw new RpcException(-100, "Unknown transaction");
+                    }
                 case "gettxout":
                     {
                         UInt256 hash = UInt256.Parse(_params[0].AsString());