Use lower connection pool defaults ⬇️

[tomchristie]

Right now we're using the default settings for the connection pool with asyncpg.
This ends up being 10 connections per worker process.
We're running 2 worker processes, so holding onto 20 database connections.
This just happens to also be the maximum number of allowed postgres connection against a Hobby-tier database, so when we run migrations we're now getting a failure with "Too many connections".