previously, the biggest file that could be uploaded through
cloudflare was 383 GiB, due to max num chunks being 4096
`--u2sz`, which takes three ints (min-size, target, max-size)
can now be used to enforce a max chunksize; chunks larger
than max-size gets split into smaller subchunks / chunklets
subchunks cannot be stitched/joined, and subchunks of the
same chunk must be uploaded sequentially, one at a time
if a subchunk fails due to bitflips or connection-loss,
then the entire chunk must (and will) be reuploaded
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