oobmigration: Set minimum delay when registering with conf

Created by: efritz

This is a refactor step towards #39578 (closed). This PR sets the interval between migration runner up/down calls to a nanosecond (pass zero uses a default of 1s). The migrator will be the only caller constructing runners with a canned config/store factory, and there's no reason to delay migration runs in this scenario as there's no active instance to disrupt.

Test plan

Existing CI pipelines.