Enhance Concurrency Adjuster to take Request Handler Idle Percent into account

[efeg]

Cruise Control's Concurrency Adjuster automatically sets up the inter-broker replica reassignment and leadership concurrency for executions (see #1288 and #1298).

The current automation is based on a feedback loop that uses selected broker metrics -- i.e. it uses the relevant broker metrics to decide whether to increase, decrease, or maintain the current pace of reassignment. However, we observed that under certain cases while broker metrics are within an acceptable range, the cluster might have a lower than acceptable Request Handler Idle Percent (aka. Request Handler Idle Pool Ratio or RHPIR) for some brokers.

Take the Request Handler Idle Percent into account while making decisions to increase, decrease, or maintain the current pace of reassignment.