Add support for various PostgreSQL settings
This adds support for:
- random_page_cost: https://gitlab.com/gitlab-com/infrastructure/issues/1630
- max_locks_per_transaction: https://gitlab.com/gitlab-com/infrastructure/issues/1559
- log_temp_files: https://gitlab.com/gitlab-com/infrastructure/issues/1557
- log_checkpoints: https://gitlab.com/gitlab-com/infrastructure/issues/1553