storage: convert some blocking i/o to non-blocking

[syphar]

When working on something else I discovered that there were places where I used blocking i/o functionality inside async functions.

It wasn't too big of an issue, since this is only used on the build-servers, so blocking the tokio runtime threads is not a huge problem, but for other things I'm planning it would be good to fix ( streaming / parallel uploads).

I now validated that all remaining sync i/o is inside spawn_blocking.

For one function (upload a whole local folder) I also now introduced paralellism.

The change to our own spawn_blocking was necessary so we can use it in the same way as the original tokio spawn_blocking ( = use the function to spawn a task, that then start running, and join it later).

[syphar]

@GuillaumeGomez tests pass now

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.