Support Postgres Plans with Separated Storage (#36)

* Add more plans

* Move to separated plans

- Testers will no longer be able to generate legacy Postgres plans

* Use downgrade to free as destructive change

- Previously, we relied upon moving to a smaller plan which used to be
illegal

* Allow for the traditional Postgres port in our external connection strings

* Appease pre-commit

* Update README.md with sample values

* Generate new cassettes