Scaling operations should be cancellable

[otrosien]

Expected Behavior

The ES Operator could decide to scale down, but while scaling down the thresholds for scale up are being exceeded, e.g. we're reaching a point where we should stop scaling down, or even scale back up.

Actual Behavior

At the moment the scaling operation is bound to finish before a new scaling operation can be started.

Steps to Reproduce the Problem

1. create an EDS where scale up and scale down thresholds are close by
2. put some more data on the nodes to slow down the scaling operation
3. scale-down and watch out for "Scaling hint: UP" during the scale down operation