TestBlobCache improvements.

This PR renames `TestBlobCache` to `InMemoryBlobCache` and makes the `InMemoryBlobCache` implement the `IObjectBlobCache` interface and the `IBulkObjectBlobCache` interface.

## Other improvements

* GetAllKeys() now returns an IEnumerable.
* ServiceProvider is gone.
* Added ExceptionHelper to make the exceptions consistent.

## TODO

- [ ] Actually add `IBulkObjectBlobCache` to the `InMemoryBlobCache`.