regularly.
Also, convert qps_test to test, from benchmark

regularly.
Also, convert qps_test to test, from benchmark

Removed registry for benchmark reports & introduced benchmark_config.{h,cc} in the spirit of test_config.{h,cc}.

The purpose of benchmark_config is to allow for different behaviors to
be decided at compile-time.

Still TODO:
- a way to plug a metadata processing (somewhat elsewhere but did not
  one to overload this already large PR).
- plug-in the auth context on the client side.
- Better end to end testing.

Port fling_test to it.

This will be used to:
- port remaining tests to Windows
- enable testing what happens when servers or clients mysteriously disappear

Move completion queue binding for new requests to the new request
request time, not server instantiation time.