Fix request cancellation when frontend worker is busy

[colega]

Describe the bug

When we try to cancel an enqueued request:

mimir/pkg/frontend/v2/frontend.go

Lines 227 to 236 in e2e2e10

	select {
	case <-ctx.Done():
	if cancelCh != nil {
	select {
	case cancelCh <- freq.queryID:
	// cancellation sent.
	default:
	// failed to cancel, ignore.
	}
	}

We have a default clause which we will follow if worker is busy sending or cancelling another request:

http://github.com/grafana/mimir/blob/e2e2e100ec14e7707d1733908e61c83c8b87ef40/pkg/frontend/v2/frontend_scheduler_worker.go#L285-L286

So sometimes the requests are processed in the queriers even when the client has canceled the request.

This is the same as fixed in loki in grafana/loki#5113

We should apply a similar fix.

To Reproduce

See test in grafana/loki#5113

Expected behavior

All upstream requests should be canceled when downstream cancels the originating request.