Avoid writing zarr chunks to temp files

[will-moore]

Currently we write zarr chunks to a local file and then serve that file:

omero-web-zarr/omero_web_zarr/views.py

Line 258 in 67bb245

with tempfile.TemporaryDirectory() as tmpdirname:

This is because I didn't know how to stream it directly in memory, with correct size header etc.
Would be nice to work out how to do that, since I would expect it to be faster and cleaner.