• Fixed a bug that could lead to a crash in main connection output handling.
  This occured only when the last request on a HTTP/2 connection had been
  processed and the session decided to shut down. This could lead to an attempt
  to send a final GOAWAY while the previous write was still in progress.
  See PR 66646.
• added --enable-debug to configure which adds rutime checks and asserts,
  useful for development. Not recommended for production use.
• fixed error handling when flushing output on main connection.