LP: #1646303 A few additional notes: This branch reverts the hard linking of cache files, as the cached file needs an immutable hash and should not change when a new snap is built. A new error code, `processing_upload_delta_error` has been introduced, which the SCA notification endpoint will return in the event of a delta reconstitution failure. In the event of this error, or the client fails to generate a delta, snapcraft will fall back to the normal upload path. The branch also introduces a new dependency, pysha3, which is required for calculating sha3_384 hashes. While this can be slow across large files, a later optimisation would be skipping the source snap hash calculation once snaps are cached by their sha3_384 hash rather than revision. As a small optimisation, deltas will not be uploaded and fallback if they are 90% or over the size of the target snap. Additionally, this branch re-implements snap caching to use sha3-384 hashes exclusively, instead of parsing snap filenames for revisions which was potentially brittle.