message: Add flush() to front-end messages
  The PostgreSQL wire protocol has a "Flush" message, which can be used by
  the clients for long-lived connections. Add a flush() helper for it.
diff: @@ -271,6 +271,12 @@ where
  })
  }
  +#[inline]
  +pub fn flush(buf: &mut BytesMut) {
  + buf.put_u8(b'H');
  + write_body(buf, |_| Ok::<(), io::Error>(())).unwrap();
  +}
  +
  #[inline]
  pub fn sync(buf: &mut BytesMut) {
  buf.put_u8(b'S');
  