Fixes oppia#7496: consolidate threads and process (oppia#7591)

* Draft version - multiprocessing within classes.

Note: check_codeowner has been commented.

* draft pipe stdout

* draft

* File cache

* Fix lint issues and add some dummy comments for now

* Add azure pipeline

* Cd path

* pass process of _lint_all_files

* fix lint

* fix lint

* Update azure-pipelines.yml for Azure Pipelines

* Update azure-pipelines.yml for Azure Pipelines

* test

* fix

* move

* Clean up

* Remove redundant comments and other format issues

* fix error not caught

* Fix lint issue

* Fix linter issue