cleanup(spanner): prefer spanner::Timestamp streaming (#5951)
cleanup(spanner): prefer spanner::Timestamp streaming (#5951)

Prefer streaming `google::cloud::spanner::Timestamp` values directly
over converting via `google::protobuf::util::TimeUtil::ToString()`.
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp