Consider getting rid of zookeeper as a coordinator

[michal-harish]

The purpose of the coordinator is to allow nodes to come online and offline dynamically. This is achieved by the following 3 functions:

1. inform about nodes coming online/offline and mediate their addresses to each other
2. generate assignment for each keyspace based on required replication factor and a state of previous assignment (to move as few partitions as possible)
3. elect leaders for online replicas of the same partition

[michal-harish]

this could be done by a combination of gossip/raft and single-partition kafka topic