Add parallel_fetch toggle for virtual member index files (#165)

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>

migrations/0007_virtual_parallel_fetch.sql

@@ -0,0 +1 @@
+ALTER TABLE virtuals ADD COLUMN IF NOT EXISTS parallel_fetch BOOLEAN NOT NULL DEFAULT FALSE;