Check channel updates in parallel

[gyscos]

Problem you are trying to solve

In part thanks to cargo-msrv, I have ~20 (and growing!) pinned stable versions of rust installed through rustup.
Every time I run rustup update, it seems to sequentially check for any update on each of these 20 versions, making this operation much slower than with only a couple channels.

I believe checking each of these 20 channels in parallel might improve the situation.

Here is a recording of running rustup update:

https://asciinema.org/a/yo7PjqQRII2YCrvG1UEstTmrc

Solution you'd like

Rather than checking each channel sequentially, checking them in parallel would improve the runtime of the command.

Note that this proposal is mostly for checking for updates. I do not suggest actually downloading and installing updates in parallel, though it might be an option as well.

Notes

No response

[rbtcollins]

Completely agree. In fact we'd like to move the core to async, but noone has started on this yet.

[r3bu1ld3r]

Hi! If you don't mind, I'd like to work on this

[djc]

@r3bu1ld3r sounds great, let us know if you need any guidance.