decouple composefs and zstd:chunked

[cgwalters]

Today,

[storage.options.overlay]
use_composefs = "true"

Actually also requires:

[storage.options]
pull_options = {convert_images = "true"} 

And it's especially the convert_images = true that adds huge latency to pulls of images that aren't already in zstd:chunked format.

I am probably willing to put some time in code behind decoupling these two things, as I do not believe at a technical level they need to be coupled.

If someone feels really strongly, it's OK to close this issue though.

In the meantime at a practical level, this issue will just be a reference point for what is likely to be a FAQ of "why does enabling composefs slow down my image pulls"?