The goal here is to get interactive progress on pulling,
as that can be slow and also I'd like to get more information there.

Signed-off-by: Colin Walters <[email protected]>

This more than doubles the copy phase speed for me. We
rely on a final fsync/flush of the disks when unmounting.

Signed-off-by: Colin Walters <[email protected]>

indicatif has nice support for multiple bars. Instead
of hand-rolling a `[nn/NN]` for the overall layer count,
change things so that we have:

```
Fetching layers [bar...] 8/65
 ostree chunk sha256:29fc11ff03e4b3 [bar] (0 B/s)
```

Signed-off-by: Colin Walters <[email protected]>