user_ips is kinda big, so really we want to add the index in the background
once we're running. Replace the schema delta with one which will do that.
I've done this in a way that's reasonably easy to reuse as there a few other
indexes I need, and I don't suppose they will be the last.
The progress for each background update is stored as a JSON blob in the
database. Each background update is broken up into separate batches.
The batch size is automatically tuned to try avoid blocking single
threaded databases for too long.
