Some testing scenarios (e.g. when testing containers using podman
services) require using a different host for the "local" instance.

This reverts commit 7cf52a2a.

It turns out that the LudicrousDB documentation is slightly misleading,
and the 'dataset' callback actually receives exactly the same arguments as
with HyperDB, so there is no need for custom switching code.

It is a replacement for HyperDB.

Controlled by defining R2DB_USE_LUDICROUSDB to non-null in db-config.php.

Instead of using an attribute inside $wpdb.

The configuration has been refactored to be a JSON file. This is
the codebase that has been running in production for the last 3
years, from ai3/docker/noblogs.

Let's make this stupid and simple for the time being. We can think
later about if and how we want to improve this!