Turn off PostgreSQL fsync to speed up CI

I'm wondering if we can significantly improve our CI test runs by setting fsync=off:

• https://pythonspeed.com/articles/faster-db-tests/
• https://www.postgresql.org/docs/current/non-durability.html

https://github.com/docker-library/postgres/issues/149 and https://github.com/docker-library/postgres/pull/54#issuecomment-190978193 mention this can only be set by the command-line. I'm not sure if I see a way to run this in CI without having our own image:

• http://gilgamezh.me/en/posts/postgres-non-durable-options-docker-container/

Thoughts @NikolayS?