RpcClient#shutdown 极端情况可能会出现hang死情况

**RpcClient**建立连接过程中是同步等待在这里：
```
at io.netty.util.concurrent.DefaultPromise.awaitUninterruptibly(DefaultPromise.java:286)
- locked <0x00000006506433d0> (a io.netty.channel.DefaultChannelPromise)
at io.netty.channel.DefaultChannelPromise.awaitUninterruptibly(DefaultChannelPromise.java:135)
at io.netty.channel.DefaultChannelPromise.awaitUninterruptibly(DefaultChannelPromise.java:28)
at com.baidu.jprotobuf.pbrpc.transport.ChannelPoolObjectFactory.wrap(ChannelPoolObjectFactory.java:75)
at com.baidu.jprotobuf.pbrpc.transport.ChannelPoolObjectFactory.wrap(ChannelPoolObjectFactory.java:35)
at org.apache.commons.pool2.BasePooledObjectFactory.makeObject(BasePooledObjectFactory.java:60)
at org.apache.commons.pool2.impl.GenericObjectPool.create(GenericObjectPool.java:868)
at org.apache.commons.pool2.impl.GenericObjectPool.borrowObject(GenericObjectPool.java:435)
at org.apache.commons.pool2.impl.GenericObjectPool.borrowObject(GenericObjectPool.java:363)
at com.baidu.jprotobuf.pbrpc.transport.ChannelPool.getChannel(ChannelPool.java:61)
at com.baidu.jprotobuf.pbrpc.transport.RpcChannel.getConnection(RpcChannel.java:64)
at com.baidu.jprotobuf.pbrpc.client.ProtobufRpcProxy.invoke(ProtobufRpcProxy.java:366)

```
如果上述连接未建立完成/连接失败，并且**RpcClient**过早暴露并被其他线程执行**shutdown**，上面的线程无法再被唤醒。

无法被唤醒的具体原因是依赖的**Netty 4.0.27.Final** shutdown 的时候没有处理未完成连接的channel等待的**DefaultChannelPromise**。最新版的Netty已经修复这个bug。

### Bug复现：
#### 环境信息

* Jprotobuf-rpc-socket:4.1.0
* 操作系统版本: Debian 4.19.98-1
* JDK版本：Java HotSpot(TM) 64-Bit Server VM 18.9 (build 11.0.5+10-LTS, mixed mode)
```
import com.baidu.jprotobuf.pbrpc.EchoService;
import com.baidu.jprotobuf.pbrpc.client.ProtobufRpcProxy;
import com.baidu.jprotobuf.pbrpc.transport.RpcClient;
import org.junit.Test;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

/**
* Coded by senninha on 2020/5/12
*/
public class TestRpcHang {
@Test
public void testRpcHang() throws InterruptedException {
while (true) {
ExecutorService executorService = Executors.newSingleThreadExecutor();
final RpcClient rpcClient = new RpcClient();
// 创建EchoService代理
ProtobufRpcProxy pbrpcProxy = new ProtobufRpcProxy(rpcClient, EchoService.class);
pbrpcProxy.setPort(1080);
pbrpcProxy.setLookupStubOnStartup(true);
executorService.submit(new Runnable() {
@Override
public void run() {
rpcClient.shutdown();
}
});
try {
// 动态生成代理实例
EchoService echoService = pbrpcProxy.proxy();
} catch (Exception e) {
// ignore
}
System.err.println("Not hang!");
Thread.sleep(1000);
}
}
}
```

纯Netty版复现：
```
@Test
public void testNetty() {
ExecutorService executorService = Executors.newSingleThreadExecutor();
while (true) {
final Bootstrap bootstrap = new Bootstrap();
final NioEventLoopGroup group = new NioEventLoopGroup(1);
bootstrap.group(group);
bootstrap.channel(NioSocketChannel.class);
bootstrap.option(ChannelOption.SO_KEEPALIVE, true);
bootstrap.option(ChannelOption.CONNECT_TIMEOUT_MILLIS, 1000);
bootstrap.handler(new ChannelInitializer() {
@Override
public void initChannel(SocketChannel ch) {
ch.pipeline().addLast(new TimeClientHandler());
}
});
ChannelFuture connect = bootstrap.connect("localhost", 1080);
executorService.submit(new Runnable() {
@Override
public void run() {
group.shutdownGracefully();
}
});
try {
connect.awaitUninterruptibly();
} catch (Exception e) {
e.printStackTrace();
}
}
}

private static class TimeClientHandler extends ChannelInboundHandlerAdapter {
@Override
public void channelRead(ChannelHandlerContext ctx, Object msg) {
ByteBuf m = (ByteBuf) msg; // (1)
try {
long currentTimeMillis = (m.readUnsignedInt() - 2208988800L) * 1000L;
System.out.println(new Date(currentTimeMillis));
ctx.close();
} finally {
m.release();
}
}

@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
cause.printStackTrace();
ctx.close();
}
}

```

Start with ChannelPoolObjectFactory.wrap, ChannelPool.getChannel, RpcChannel.getConnection, and RpcClient.shutdown from the stack trace. Run the TestRpcHang or testNetty reproductions under the stated Java and Jprotobuf-rpc-socket versions. Done means shutdown during an incomplete connection no longer leaves the waiting thread blocked indefinitely.

Written by the indexing model from the issue text.