remove sync from channel closing (#118)

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clients/java/jsonrpc/jsonrpc-client/src/main/java/org/kurento/jsonrpc/client/JsonRpcClientNettyWebSocket.java

@@ -19,6 +19,7 @@
 import java.io.IOException;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.TimeoutException;
+import java.util.concurrent.Future;
 import java.util.concurrent.locks.Condition;
 import java.util.concurrent.locks.Lock;
 import java.util.concurrent.locks.ReentrantLock;
@@ -356,11 +357,11 @@ public void closeNativeClient() {
     handler = null;
   }
 
-  private void closeChannel() {
+  private Future<Void> closeChannel() {
     if (channel != null) {
       log.debug("{} Closing client", label);
       try {
-        channel.close().sync();
+        return channel.close();
       } catch (Exception e) {
         log.debug("{} Could not properly close websocket client. Reason: {}", label, e.getMessage(),
             e);
@@ -369,6 +370,7 @@ private void closeChannel() {
     } else {
       log.warn("{} Trying to close a JsonRpcClientNettyWebSocket with channel == null", label);
     }
+    return null;
   }
 
 }