Add client functionalities to address issue: https://github.com/allartk/leaflet.offline/issues/314

[disarticulate]

Export openTilesDataBase function to gain access to the underlying db/store and add a createdAt index to allow clients to prune based on how old tiles are.

Does not:

• Address any upgrades required for existing clients with the new index. You may need to invoke versioning to add the index to existing clients. Clients would need to figure out how to add that index themselves if they wanted to make a smooth transition.
• Provide any additional functionality to search keys, so this would just allow for further improvements to caching.