[TEST][Monitoring] Move payload = null above call (elastic/x-pack-elasticsearch#4053)

This moves the `payload = null;` statement to above the asynchronous
HTTP call. This helps to avoid a race condition relative to `doClose`
asserting that it is `null`.

This is only a realistic situation during a shutdown situation because
the thread responds immediately before it can be nullified and freeable,
which is not a realistic scenario in any other situation.

Original commit: elastic/x-pack-elasticsearch@eb3a6ff118

plugin/monitoring/src/main/java/org/elasticsearch/xpack/monitoring/exporter/http/HttpExportBulk.java

@@ -96,6 +96,9 @@ class HttpExportBulk extends ExportBulk {
         } else if (payload.length != 0) {
             final HttpEntity body = new ByteArrayEntity(payload, ContentType.APPLICATION_JSON);
 
+            // free the memory
+            payload = null;
+
             client.performRequestAsync("POST", "/_bulk", params, body, new ResponseListener() {
                 @Override
                 public void onSuccess(Response response) {
@@ -115,9 +118,6 @@ class HttpExportBulk extends ExportBulk {
                     }
                 }
             });
-
-            // free the memory
-            payload = null;
         }
     }