uploader sometimes chokes on big chunks

[jeremybmerrill]

if there's a

#<Manticore::SocketTimeout: Read timed out> Upload error: Read timed out Read timed out.

then we should cut the slice_size in half or something

[jeremybmerrill]

fixed with slice-size stuff ehre: https://github.com/newsdev/stevedore-uploader/commit/6948fe58dcfa4d839cd8e0b96c65b3abfb1d8c0f