title: RestHighLevelClient with BulkProcessor Elastic Search issue
body: <p>Earlier I was using TransportClient in my app.
  Recently moving towards AWS manages Elastic Search services.
  Learned that AWS managed ES Cluster would not support TransportClient.</p>
  
  <p>So migrating the code where it was using BulkProcessort to insert documents to ES.
  When I refactor the code as a part of ES documentation I added this line.</p>
  
  <pre><code>BulkProcessor bulkProcessor = BulkProcessor.builder(client::bulkAsync, listener).build(); 
  </code></pre>
  
  <p>and I get an error at client::bulkAsync saying Client is not a functional interface. </p>
  
  <p>Need help understanding what am I doing wrong.</p>
  
  <p>Document Link For reference,</p>
  
  <p><a href="https://www.elastic.co/guide/en/elasticsearch/client/java-rest/master/java-rest-high-document-bulk.html#java-rest-high-document-bulk-processor" rel="nofollow noreferrer">https://www.elastic.co/guide/en/elasticsearch/client/java-rest/master/java-rest-high-document-bulk.html#java-rest-high-document-bulk-processor</a></p>