Cache for a BlobService, using a “near” and “far” blobservice.
Requests are tried in (and returned from) the near store first, only if
things are not present there, the far BlobService is queried.
In case the near blobservice doesn’t have the blob, we ask the remote
blobservice for chunks, and try to read each of these chunks from the near
blobservice again, before falling back to the far one.
While doing so, the full blob is written to the near blobservice.
The far BlobService is never written to.