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>

pkg/models/virtual.go

@@ -3,11 +3,12 @@ package models
 import "time"
 
 type Virtual struct {
-	Name        string      `json:"name"`
-	PackageType PackageType `json:"package_type"`
-	Description string      `json:"description,omitempty"`
-	Members     []string    `json:"members"`
-	ManagedBy   string      `json:"managed_by,omitempty"`
-	CreatedAt   time.Time   `json:"created_at"`
-	UpdatedAt   time.Time   `json:"updated_at"`
+	Name          string      `json:"name"`
+	PackageType   PackageType `json:"package_type"`
+	Description   string      `json:"description,omitempty"`
+	Members       []string    `json:"members"`
+	ManagedBy     string      `json:"managed_by,omitempty"`
+	ParallelFetch bool        `json:"parallel_fetch"`
+	CreatedAt     time.Time   `json:"created_at"`
+	UpdatedAt     time.Time   `json:"updated_at"`
 }