Destroying blob stores

I really like abstract-blob-store and am using a few different blob stores currently (like fs- and idb-), but I'm a little stumped on the clean-up aspect. For example, fs-blob-store will happily mkdir when you init the module, but there's no general API for deleting the blob store -- just its keys. 😞

This means clean-up must be implementation-specific (since your code will need to know whether to rimraf or to delete the index-db store, etc.) to prevent lingering side-effects of the blob store, such as extra directories hanging around your FS or keys in your IndexedDB store.

This may be best served as a new API method on abstract-blob-store (maybe destroy?). Eager to get your thoughts on this @maxogden and @mafintosh (and anyone else excited about side effect free modules!).

Review the existing abstract-blob-store interface alongside the fs-blob-store and idb-blob-store implementations mentioned in the issue. Determine the expected destroy behavior for each backend and define how completion would verify that filesystem directories or IndexedDB stores no longer remain.