This allows for better compiler errors, where the compiler will state a
copy or move couldn't occur due to the relevant function being deleted.
Previously a compiler would warn about the relevant function not being
accessible (which, while true, isn't as informative as it could be).
CallbackStatus instances aren't the cheapest things to copy around
(relative to everything else), given that they're currently 520 bytes in
size and are currently copied numerous times when callbacks are invoked.
Instead, we can pass the status by const reference to avoid all the
copying.
german77