Include ISR Information in Cluster Model

[Arc676]

This is a feature request in two parts¹:

1. Include ISR state information in Cruise Control's cluster model. This will allow goals to also consider ISR state before making recommendations.
2. Extend the rack awareness goal with the option to make recommendations based on ISR state. The objective would be to ensure that in-sync replicas are spread out among as many racks as possible.

This feature should be opt-in; the first part of this feature could drastically increase Cruise Control's memory footprint.

The extension of the rack awareness goal could also be implemented as a separate goal. This would ensure a cleaner separation of the goals' objectives and make it simpler to skip this new goal when the first feature is disabled.

See original discussion in #2273.

Footnotes

1. If preferred, this can be split into two issues. ↩