This covers the functionality tests for streaming in-progress
subtransactions, streaming transactions containing rollback to savepoints,
and streaming transactions having DDLs.

Author: Tomas Vondra, Amit Kapila and Dilip Kumar
Reviewed-by: Dilip Kumar
Discussion: https://postgr.es/m/688b0b7f-2f6c-d827-c27b-216a8e3ea700@2ndquadrant.com