Ask HN: What distributed DB is best for a Q&A site?

I'm building a Q&A site. Would like to use a distributed DB for data store.

Requirements:

1) Elastic: add nodes and scale out automatically

2) HA: automatically & transparently provides replication & failover

3) Locality: I'd like to group all answers to the same question together and put them in the same shard. That way when retrieving answers to a question I won't need to query all nodes

4) Good single-node performance

5) Secondary index (?) - each answer has a score. I'd like to sort answers by their scores. Something like a secondary index in RDBMS

Most NoSQL I know are K/V stores that don't support (3). What would you recommend?