Use buildbox-casd for CAS access

buildbox-fuse is currently a standalone component using direct read/write filesystem access to the local CAS cache and built-in support for downloading and uploading blobs from/to CAS servers.

To properly support a shared local CAS cache with LRU expiry, direct filesystem access must be limited to read-only operations. All write access must be handled by casd. Downloading and uploading should also be handled by casd to allow better handling of concurrent transfers and avoid code duplication.