SQL DSL stage name should probably be named application nodes

Currently, the SQL DSL expansion uses its own custom out-of-band way of assigning hierarchical names to the transforms. But Beam already has hierarchical transforms with names, and these are intended for use in APIs and UIs. We should see what the diff is between just using Beam's existing facilities and this SQL DSL "stage name" concept.

Imported from Jira [BEAM-3716](https://issues.apache.org/jira/browse/BEAM-3716). Original Jira may contain additional context.
Reported by: kenn.

The issue names no files, tests, or entry points. Start by comparing the SQL DSL expansion's custom hierarchical stage-name mechanism with Beam's existing hierarchical transform facilities; done means documenting the differences and determining whether the existing facilities can replace the custom concept.

Written by the indexing model from the issue text.