Support concurrent DDL

This issue is used to keep tracking of the development of "concurrent DDL"

Ref #18396

SPEC

SPEC describes what we want to do exactly from the user's perspective.
You can leave your comment on the document if you have any suggestions or questions.
Chinese version: https://docs.google.com/document/d/1s1ju_Ubmo2aEIF5hDWBBALnqZ2CHKNiFAaBp85NyRXQ/edit#
English version: https://docs.google.com/document/d/1e6B-GIIZn3elN02ub4jkCyqcUhcjUAKKaDAAphkg2ss/edit#