Race condition if two worker instances attempt to initialize clean database in parallel

Only affects first run of worker against your DB, so if you already have worker running you're fine.

If you run 10 `graphile-worker` instances against a clean database all at the same time there's a good chance that in this code:

https://github.com/graphile/worker/blob/436e29968d28c48d5f46a012b9c83b0747e2eff1/src/migrate.ts#L117-L137

two or more will generate the requisite `42P01`/`42703` error, and both will attempt to install the schema, and only one can succeed. The other will throw an error and exit.

There's two bugs here:

1. the `installSchema` should be wrapped with `try/catch` so that errors thrown there will trigger a retry of the whole loop
2. there's no `sleep()`, so it'll instantly retry which can be painful - we should use randomized back-off