Switch to launchctl for Postgres

`brew services` is no longer supported:

https://github.com/Homebrew/homebrew/commit/c0b99c031fec0a2096b85a458d7b42e3627560d8

Also fixes #93 and #243.

Idempotently creates Postgres database cluster and starts (or restarts)
Postgres.
Does the `&` here start both of these in the background? If you start and background both at once, is there a potential race condition here?