Virtual members already merge concurrently, but each rpm member fetches primary/filelists/other one after another, and each deb member its Packages files the same way. On large virtuals the slowest member's serial chain pushes the merge past dnf's 30s timeout.
- add `parallel_fetch` (default false) to virtuals: model, v2 API, migration 0007
- fetch a member's index files concurrently (up to 4) when set; first error still fails the member
- thread the flag through rpm and deb member fetches
Reviewed-on: #165
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>
