PG: join new vertex data by vertex ids (#2796)

Fixes #2793
Fixes #2794

We are using the assumption that there should be a single row for each vertex id.

This does not yet handle MG.  Let's figure out how we want SG to behave first.

Authors:
  - Erik Welch (https://github.com/eriknw)

Approvers:
  - Brad Rees (https://github.com/BradReesWork)
  - Vibhu Jawa (https://github.com/VibhuJawa)

URL: #2796