Managed storage can be inaccessible

[smalluban]

According to our investigation (related to #2814) and found issues in other extensions (w3c/webextensions#497 (comment)), the storage.managed can be inaccessible on SW startup.

To mitigate the issue, we should consider:

• adding storage.local cache layer - keep the latest fetched config locally and use it as a fallback
• adding retries to fetch the managed storage, but with a reasonable timeout and retries count