More refactoring and restructuring

* Rename DALDb to BlobIndex.
* Backflip: just two packages for now.
* Backflip: PUT returns Blobs again not ids.
* Move public interfaces into doss package.
* Move implementation details into doss.local.
* Introduce the Container interface.
The word get in getId is just noise. Blob ain't no bean.
Blobs have a size
Transaction resumption