Limit the number of concurrent scale operations to reduce cluster churn

Until we move to using the upcoming Rescale API which recycles pods, we need to be mindful with how many deployments we scale at the same time because each of them is going to give up all its pods and require the new number of required pods.

This can cause churn in the cluster and temporary lead to "unallocatable" pods which triggers the k8s cluster autoscaler to add more cluster nodes. That is often not desirable because the actual required resources after the scaling have been settled, are lower.