Address some review comments about signal and restoring qps_test_with_poll

test/cpp/qps/async_streaming_ping_pong_test.cc

test/cpp/qps/async_unary_ping_pong_test.cc

test/cpp/qps/client.h

test/cpp/qps/qps_driver.cc

test/cpp/qps/qps_openloop_test.cc

test/cpp/qps/qps_test.cc

test/cpp/qps/qps_test_with_poll.cc

test/cpp/qps/qps_worker.cc

test/cpp/qps/secure_sync_unary_ping_pong_test.cc

test/cpp/qps/server.h

test/cpp/qps/server_async.cc

test/cpp/qps/server_sync.cc

test/cpp/qps/sync_streaming_ping_pong_test.cc

test/cpp/qps/sync_unary_ping_pong_test.cc