Support multi-datacenter replication

[minwoox]

We have to choose between:

• the current Multi-Leader replication.
  • We can use ZooKeeper hierarchical quorums so that one of each cluster in a data center creates the quorum.
• Leader-Follower model.

If the write performance of the replicas is acceptable when the replicas spans over 3 data centers, we can keep the current Multi-Leader structure. But it highly depends on the latency between the data centers which means if the data centers across all over the world(e.g europe, United States, Asia) it may not work well.