Improving gRPC benchmark configurations.

**Is your feature request related to a problem? Please describe.**

The benchmark of gRPC framework was collected under less optimal configurations.
1. WRT unary Echo, the client didn't leverage the ability of multiplexing requests over single HTTP/2 connection.
2. WRT streaming Echo, There is only one connection has been established, which may lead to head of line problem very quickly.

Comparing KiteX benchmark with those results may misleading developers.

**Describe the solution you'd like**

1. At least, explicitly disclose how gRPC benchmarks were collected.
2. It would be better to have a comparison with results collected under more optimal configurations.