Generate manifest file with UUID-labeled name, not a temporary file name

[jeff-cohere]

When a transfer has completed, the DTS writes a manifest file to a directory and then transfers it to it's payload's destination. Currently, this manifest file is treated as a temporary file with an autogenerated name. The name does not indicate which transfer the DTS file belongs to, so there's no way to map these files themselves to transfers.

Since each transfer is already uniquely identified by its UUID, it would be pretty easy to generate a "temporary filename" using the UUID instead of an autogenerated numeric sequence (e.g. manifest-{UUID}.json). We could also embed the transfer UUID in the manifest itself to help in cases where the file has been renamed.