Configurable max_lag for dev-vs-prod

As a follow-up to https://github.com/mozilla/picl-idp/pull/244 we should make the max-lag setting on toobusy configurable, so we can use a large value for running on travis but a better-tuned value in production.

Maybe make it a config setting, maybe just `if (env !== "production)` in the code somewhere.

Not urgent, filing this so we don't forget.