DB DNS URL config assumes running inside a Fly network

[brainlid]

Currently, there is an implied dependence on Fly.io databases because of how the DNS is modified so a "nearest" database can found. This helps when an instance is auto-deployed to a backup region other than an expected region where a Postgres primary or replica database can be found.

Backup regions can be disabled, or limited to regions where there are databases present.

The goal with a change is to enable the library to work more easily outside of Fly.io networks.