Fix BulkProcessor hangs for threads deadlocked

Author: zhangmeng0426

flink-connectors/flink-connector-elasticsearch6/pom.xml

@@ -37,7 +37,7 @@ under the License.
 
 	<!-- Allow users to pass custom connector versions -->
 	<properties>
-		<elasticsearch.version>6.3.1</elasticsearch.version>
+		<elasticsearch.version>6.8.12</elasticsearch.version>
 	</properties>
 
 	<dependencies>