code: protected void beforeSaveToDestination(Map<Identity, JsonNode> sourceDocs, Map<Identity, JsonNode> destDocs, Set<Identity> insertDocs, Set<Identity> rewriteDocs, List<JsonNode> saveDocsList) {
          // do nothing by default
          // override this in child classes to get access to docs before they are saved
      }
docs: Execute this method before docs are saved to destination (Lightblue). Modifying saveDocsList
  will affect what gets actually saved. Implementation of this method is responsible for exception handling.
  
  @param sourceDocs docs read from source
  @param destDocs corresponding docs read from destination
  @param insertDocs docs which exist in source but not in destination
  @param rewriteDocs docs which existing in both source and destination, but are different
  @param saveDocsList list of docs to save