YarnShuffleService doesn't close idle netty channel

Recently, we find our YarnShuffleService has a lot of half-open connections where shuffle servers' connections are active while clients have already closed. 

For example, from server's `ss -nt sport = :7337` output we have

ESTAB 0 0 server:7337 client:port

However, on client `ss -nt dport =: 7337 | grep server` would return nothing.

Looking at the code,  `YarnShuffleService` creates a `TransportContext` with `closeIdleConnections` set to false.

public class YarnShuffleService extends AuxiliaryService {
  ...
  @Override  protected void serviceInit(Configuration conf) throws Exception { 
    ...     
    transportContext = new TransportContext(transportConf, blockHandler); 
    ...
  }
  ...
}

public class TransportContext implements Closeable {
  ...

  public TransportContext(TransportConf conf, RpcHandler rpcHandler) {       
    this(conf, rpcHandler, false, false);  
  }
  public TransportContext(TransportConf conf, RpcHandler rpcHandler, boolean closeIdleConnections) {    
    this(conf, rpcHandler, closeIdleConnections, false);  
  }
  ...
}

Hence, it's possible the channel  may never get closed at server side if the server misses the event that the client has closed it.

I find that parameter is true for `ExternalShuffleService`.

Is there any reason for the difference here ?  Can we enable closeIdleConnections in YarnShuffleService or at least add a configuration to enable it ?