public default void notifyClose() {
        // do nothing by default
    }
Gets a callback from {@link #close()} for additional operations specific to the {@link Traversal} implementation.
A good implementation will use {@link #close()} to release resources in steps and this method to release
resources specific to the {@link Traversal} implementations.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/Traversal.java