Add option to base upscale_stabilization_period on when previous request becomes live

[deliahu]

Description

A few options (other options may exist):

• Option 1: Change the definition of upscale_stabilization_period so that it is with respect to the time the replicas from previous upscale request became live. The default would be 0. Question: would it ever make sense to be less than that?

• Option 2: Add a boolean flag which switches the behavior of upscale_stabilization_period from what it is now (absolute) to as described above (relative to replica ready).

• Option 3: add a new duration flag to specify upscale_stabilization_period with respect to replica ready, and the actual upscale_stabilization_period would be the max of this one and the existing one. A value of Null would mean don't use it. This may be confusing, especially if the default is 0s instead of Null