fix Low level Redis client shutdown issue: eventloopgroup won't be shutdown.

The eventloopgroup should be shutdown gracefully

src/main/java/org/redisson/client/RedisClient.java

@@ -138,6 +138,7 @@ public class RedisClient {
 
     public void shutdown() {
         shutdownAsync().syncUninterruptibly();
+        bootstrap.group().shutdownGracefully();
     }
 
     public ChannelGroupFuture shutdownAsync() {
@@ -150,4 +151,3 @@ public class RedisClient {
     }
 
 }
-