fix: DeleteMultipleObjects to finish even if cancelled + concurrent sets (#14038)

* Process sets concurrently.
* Disconnect context from request.
* Insert context cancellation checks.
* errFileNotFound and errFileVersionNotFound are ok, unless creating delete markers.

cmd/common-main.go

@@ -911,3 +911,32 @@ func contextCanceled(ctx context.Context) bool {
 		return false
 	}
 }
+
+// bgContext returns a context that can be used for async operations.
+// Cancellation/timeouts are removed, so parent cancellations/timeout will
+// not propagate from parent.
+// Context values are preserved.
+// This can be used for goroutines that live beyond the parent context.
+func bgContext(parent context.Context) context.Context {
+	return bgCtx{parent: parent}
+}
+
+type bgCtx struct {
+	parent context.Context
+}
+
+func (a bgCtx) Done() <-chan struct{} {
+	return nil
+}
+
+func (a bgCtx) Err() error {
+	return nil
+}
+
+func (a bgCtx) Deadline() (deadline time.Time, ok bool) {
+	return time.Time{}, false
+}
+
+func (a bgCtx) Value(key interface{}) interface{} {
+	return a.parent.Value(key)
+}