Add throttle feature into reassignment execution

[fyang13]

While in general splitting the partitions to multiple batch will reduce the chance that broker get overwhelmed by rebalance traffic, it's still possible rebalance traffic will pose pressure to brokers if cluster is busy. I would like to add the throttle to feature to add another layer of protection to prevent cluster get overrun by rebalance workload. By default it's set to "-1" so it won't be throttled unless specified.