More automated stress tests against multiple scenarios

[kailuowang]

Scenarios

1. Load balancing adding new handler
2. Load balancing losing a handler abruptly
3. Overhead should be less than 1ms
4. Overcapacity throughput should be throughput under optimal concurrency
5. Under capacity traffic should have the same throughput/latency as round robin direct through
6. Two nodes with 200 RPS and 100 RPS capacity. Make the 200RPS unresponsive(or really slow) which brings the capacity below the incoming requests.
7. Two nodes each with 300RPS capacity. One of the cluster node became out of response (or really slow) , the left node still have enough capacity to serve the incoming request