[server] Support RackAwareGoal and RackAwareDistributionGoal for rebalance

[swuferhong]

Purpose

Linked issue: close #2379

This pr aims to introduce two type of goal to rebalance for rack aware situation:

• RackAwareGoal: Generate replica movement proposals to provide rack-aware replica distribution, which ensure that all replicas of each bucket are assigned in a rack aware manner -- i.e. no more than one replica of each bucket resides in the same rack.
• RackAwareDistributionGoal: A relaxed version of RackAwareGoal. Contrary to RackAwareGoal, as long as replicas of each bucket can achieve a perfectly even distribution across the racks, this goal lets placement of multiple replicas of a bucket into a single rack.

Brief change log

Tests

API and Format

Documentation