grpc stress test: not to test concurrent > 128.
grpc stress test: not to test concurrent > 128.

To avoid RST_STREAM on some of requests before
the first response is received.

Change-Id: I5c3df64c87a6264b4094b7c1f882f159a7d329f1

cloudendpoints/esp,cloudendpoints/esp,cloudendpoints/esp,cloudendpoints/esp,cloudendpoints/esp,cloudendpoints/esp,cloudendpoints/esp,cloudendpoints/esp