Dynamically change replication throttle for an on-going rebalance

Now that we have support for replication throttle (https://github.com/linkedin/cruise-control/pull/748, thanks again @aaronlevin-stripe), I thought it would be great to have a way to change a throttle in the middle of a rebalance.

In cases where you set one throttle but later realized it wasn't satisfactory (e.g things going too slow or too unstable), it is invaluable to have a quick way to revert it.

I thought adding a new endpoint for updating throttle (or maybe extending /admin?) may be straightforward to do. What are people's thoughts on this?