PreferAvailabilityStrategy: Improve selection of segment owners after merge

PreferAvailabilityStrategy picks a winning CH after merge, but that CH is very likely to be the wrong one. We should improve our odds at preserving cache data by mixing CHs, and keeping from each node's response only the information about the segments it owns itself.

The downside is that PreferAvailabilityStrategy becomes tightly coupled with DefaultConsistentHash/ReplicatedConsistentHash, and users that want to add custom ConsistentHash implementations will have to re-implement PreferAvailabilityStrategy.