Add skip predicate to node traversal. (#4222)

Add skip predicate to node traversal. (#4222)

Sometime you want to skip specific sub-DAGs when traversing a node. For example, you might be doing a transformation with a cache, and want to skip sub-DAGs that you've already processed.

This PR would add a skipIf builder method to NodeDfsIterable, which allows the user to provide a predicate according to which nodes will be omitted from the subsequent traversal.