A few interface changes

[QingMa-gp]

trafficstars

This commit includes the following modifications:

• Expose get_common_group_tles() and choose_grouping_locus()
• Extend expression_tree_walker() to support value nodes

Here are some reminders before you submit the pull request

• [ ] Add tests for the change
• [ ] Document changes
• [ ] Communicate in the mailing list if needed
• [ ] Pass make installcheck
• [ ] Review a PR in return to support the community