some improve (sofastack#459)

jraft-core/src/main/java/com/alipay/sofa/jraft/rpc/InvokeCallback.java

@@ -25,5 +25,7 @@ public interface InvokeCallback {
 
     void complete(final Object result, final Throwable err);
 
-    Executor executor();
+    default Executor executor() {
+        return null;
+    }
 }

jraft-core/src/main/java/com/alipay/sofa/jraft/util/DirectExecutor.java

@@ -0,0 +1,38 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.alipay.sofa.jraft.util;
+
+import java.util.concurrent.Executor;
+
+/**
+ * An executor that direct run command.
+ * 
+ * @author jiachun.fjc
+ */
+public enum DirectExecutor implements Executor {
+    INSTANCE;
+
+    @Override
+    public void execute(final Runnable command) {
+        command.run();
+    }
+
+    @Override
+    public String toString() {
+        return "DirectExecutor";
+    }
+}

jraft-extension/rpc-grpc-impl/src/main/java/com/alipay/sofa/jraft/rpc/impl/GrpcClient.java

@@ -44,6 +44,7 @@
 import com.alipay.sofa.jraft.rpc.InvokeCallback;
 import com.alipay.sofa.jraft.rpc.InvokeContext;
 import com.alipay.sofa.jraft.rpc.RpcClient;
+import com.alipay.sofa.jraft.util.DirectExecutor;
 import com.alipay.sofa.jraft.util.Endpoint;
 import com.alipay.sofa.jraft.util.Requires;
 import com.alipay.sofa.jraft.util.Utils;
@@ -101,20 +102,12 @@ public void registerConnectEventListener(final ReplicatorGroup replicatorGroup)
     public Object invokeSync(final Endpoint endpoint, final Object request, final InvokeContext ctx,
                              final long timeoutMs) throws RemotingException {
         final CompletableFuture<Object> future = new CompletableFuture<>();
-        invokeAsync(endpoint, request, ctx, new InvokeCallback() {
 
-            @Override
-            public void complete(final Object result, final Throwable err) {
-                if (err == null) {
-                    future.complete(result);
-                } else {
-                    future.completeExceptionally(err);
-                }
-            }
-
-            @Override
-            public Executor executor() {
-                return null;
+        invokeAsync(endpoint, request, ctx, (result, err) -> {
+            if (err == null) {
+                future.complete(result);
+            } else {
+                future.completeExceptionally(err);
             }
         }, timeoutMs);
 
@@ -138,26 +131,18 @@ public void invokeAsync(final Endpoint endpoint, final Object request, final Inv
         final Channel ch = getChannel(endpoint);
         final MethodDescriptor<Message, Message> method = getCallMethod(request);
         final CallOptions callOpts = CallOptions.DEFAULT.withDeadlineAfter(timeoutMs, TimeUnit.MILLISECONDS);
-        final Executor executor = callback.executor();
+        final Executor executor = callback.executor() != null ? callback.executor() : DirectExecutor.INSTANCE;
 
         ClientCalls.asyncUnaryCall(ch.newCall(method, callOpts), (Message) request, new StreamObserver<Message>() {
 
             @Override
             public void onNext(final Message value) {
-                if (executor == null) {
-                    callback.complete(value, null);
-                } else {
-                    executor.execute(() -> callback.complete(value, null));
-                }
+                executor.execute(() -> callback.complete(value, null));
             }
 
             @Override
             public void onError(final Throwable throwable) {
-                if (executor == null) {
-                    callback.complete(null, throwable);
-                } else {
-                    executor.execute(() -> callback.complete(null, throwable));
-                }
+                executor.execute(() -> callback.complete(null, throwable));
             }
 
             @Override