initial implementation.

src/core/lib/surface/completion_queue.c

src/core/lib/surface/completion_queue.h

src/proto/grpc/testing/control.proto

src/proto/grpc/testing/stats.proto

test/cpp/microbenchmarks/fullstack_fixtures.h

test/cpp/microbenchmarks/helpers.cc

test/cpp/microbenchmarks/helpers.h

test/cpp/qps/client.h

test/cpp/qps/client_async.cc

test/cpp/qps/driver.cc

test/cpp/qps/qps_json_driver.cc

test/cpp/qps/report.cc

test/cpp/qps/report.h