message: core: avoid flushing after ever line
  Instead just print newlines and let the OS take care of when to flush.
  This should save some CPU cycles waiting for the flush.
diff: @@ -101,7 +101,7 @@ public:
  std::cout << _s.str();
  std::cout << " (" << _caller_filename << ":" << std::dec << _caller_filenumber << ")";
  - std::cout << std::endl;
  + std::cout << '\n';
  #endif
  }
  