Fix Version/s: 2.0 beta 1
When read_repair is not 1.0, we send the request to one node for some of the requests. When a node goes down or when a node is too busy the client has to wait for the timeout before it can retry.
It would be nice to watch for latency and execute an additional request to a different node, if the response is not received within average/99% of the response times recorded in the past.
CASSANDRA-2540 might be able to solve the variance when read_repair is set to 1.0
1) May be we need to use metrics-core to record various Percentiles
2) Modify ReadCallback.get to execute additional request speculatively.