code: protected String getMasterRequestId(XmlObject msgdoc, Map<String,String> metaInfo) {
          String masterRequestId = metaInfo.get(Listener.METAINFO_MDW_REQUEST_ID);
          if (masterRequestId == null)
              masterRequestId = metaInfo.get(Listener.METAINFO_DOCUMENT_ID);
          else masterRequestId = this.placeHolderTranslation(masterRequestId, metaInfo, msgdoc);
          return masterRequestId;
      }
docs: This method is invoked by handleEventMessage() to obtain master request ID.
  The default implementation does the following:
  - If "mdw-request-id" is defined in metaInfo, then takes its value
  performs place holder translation
  ({@link #placeHolderTranslation(String, Map, XmlObject)}), and returns it
  - otherwise, return the external event instance ID
  You can override this method to generate custom master request ID that
  cannot be configured the above way.
  
  @param msgdoc
  @param metaInfo
  @return