code: @Override
      public <T> Iterable<Key<T>> insert(final Iterable<T> entities) {
          return insert(entities, new InsertOptions()
                                      .writeConcern(defConcern));
      }
docs: Inserts entities in to the database
  
  @param entities the entities to insert
  @param <T>      the type of the entities
  @return the keys of entities