Memory Leak when I try to create client
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### Expected behavior
It should't have memory leak.
### Actual behavior
```
[main] ERROR io.netty.util.ResourceLeakDetector  - LEAK: HashedWheelTimer.release() was not called before it's garbage-collected. See http://netty.io/wiki/reference-counted-objects.html for more information.
Recent access records: 
Created at:
	io.netty.util.HashedWheelTimer.<init>(HashedWheelTimer.java:272)
	io.netty.util.HashedWheelTimer.<init>(HashedWheelTimer.java:216)
	io.netty.util.HashedWheelTimer.<init>(HashedWheelTimer.java:195)
	org.redisson.connection.MasterSlaveConnectionManager.initTimer(MasterSlaveConnectionManager.java:318)
	org.redisson.connection.MasterSlaveConnectionManager.<init>(MasterSlaveConnectionManager.java:161)
	org.redisson.connection.SingleConnectionManager.<init>(SingleConnectionManager.java:34)
	org.redisson.config.ConfigSupport.createConnectionManager(ConfigSupport.java:192)
	org.redisson.Redisson.<init>(Redisson.java:122)
	org.redisson.Redisson.create(Redisson.java:159)
	test.main(test.java:32)
```

### Steps to reproduce or test case
```java=
        Codec stringCodec = new StringCodec();

        for (int i = 0; i < 10; i++) {
            RedissonClient client;
            Config config = new Config();
            config.useSingleServer().setAddress(RedisConfig.Address);
            config.useSingleServer().setPassword(RedisConfig.Password);
            config.setCodec(stringCodec);
            client = Redisson.create(config);

            BatchOptions options = BatchOptions.defaults();
            RBatch pipe = client.createBatch(options);

            pipe.getBucket("test", stringCodec).getAsync();
            BatchResult res = pipe.execute();

            System.out.println(res.getResponses().get(0));

            client.shutdown();

            System.gc();
        }

```

### Redis version
4.0.10
### Redisson version
3.7.5
### Redisson configuration
```
Config config = new Config();
config.useSingleServer().setAddress(RedisConfig.Address);
config.useSingleServer().setPassword(RedisConfig.Password);
config.setCodec(stringCodec);
client = Redisson.create(config);
```
