Refactoring std::chrono out.

include/grpc++/client_context.h

include/grpc++/completion_queue.h

include/grpc++/credentials.h

include/grpc++/server_context.h

include/grpc++/time.h

src/cpp/client/channel.cc

src/cpp/client/client_context.cc

src/cpp/client/secure_credentials.cc

src/cpp/common/completion_queue.cc

src/cpp/server/server.cc

src/cpp/server/server_context.cc

src/cpp/util/time.cc

test/cpp/client/credentials_test.cc

test/cpp/end2end/async_end2end_test.cc

test/cpp/end2end/end2end_test.cc

test/cpp/end2end/generic_end2end_test.cc

test/cpp/interop/client.cc

test/cpp/util/time_test.cc