closedts: react faster to config changes

This approximately halves the duration of

`make test PKG=./pkg/ccl/changefeedccl TESTS=/sinkless`,

from ~60s to ~30s.

Touches #34455.

Release note: None

pkg/storage/closedts/provider/provider.go

@@ -119,6 +119,15 @@ func (p *Provider) runCloser(ctx context.Context) {
 	ch := p.Notify(p.cfg.NodeID)
 	defer close(ch)
 
+	confCh := make(chan struct{}, 1)
+	confChanged := func() {
+		select {
+		case confCh <- struct{}{}:
+		default:
+		}
+	}
+	closedts.TargetDuration.SetOnChange(&p.cfg.Settings.SV, confChanged)
+
 	var t timeutil.Timer
 	defer t.Stop()
 	var lastEpoch ctpb.Epoch
@@ -134,6 +143,9 @@ func (p *Provider) runCloser(ctx context.Context) {
 			return
 		case <-t.C:
 			t.Read = true
+		case <-confCh:
+			// Loop around to use the updated timer.
+			continue
 		}
 
 		next, epoch, err := p.cfg.Clock(p.cfg.NodeID)