The new lifetime for the store is: - `init` constructs it, however it can not be used yet. - `#enable` flags it as ready to be used. - `#disable` closes any DB connections, then flags it as not ready. - `uninit` destroys the it. Created a new class to hold the `RustRelevancyStore`. This allows us to test the logic there without needing to fiddle with prefs, nimbus, or timers. Differential Revision: https://phabricator.services.mozilla.com/D211254