induce delay prior to scale down (#3381)

This is to avoid a narrow race between adding server and them catching
up where they also register as current.

Also wait for all peers to be caught up.

This also avoids clearing catchup marker once catchup stalled.
A stalled catchup would remove the marker causing the peer to
register as current.

Signed-off-by: Matthias Hanel <mh@synadia.com>

server/jetstream_super_cluster_test.go

@@ -1812,7 +1812,7 @@ func TestJetStreamSuperClusterMovingStreamsAndConsumers(t *testing.T) {
 
 			// Should see the cluster designation and leader switch to C2.
 			// We should also shrink back down to original replica count.
-			checkFor(t, 10*time.Second, 100*time.Millisecond, func() error {
+			checkFor(t, 20*time.Second, 100*time.Millisecond, func() error {
 				si, err := js.StreamInfo("MOVE")
 				if err != nil {
 					return err