Serialization of regular expressions doesn't add escaping slashes

The patterns used to store the value will contain the raw string passed to the ParamRegex class when the correct thing to do would be to store the unescaped pattern and add the escaping characters during serialization. The lack of escaping slashes has been deprecated for a long time so this is mostly a cosmetic thing until the handling of it is removed.

This also fixes the practical semantics of the mxs::RegexValue::pattern() to again correctly return the pattern and not the raw configuration value.