QC: Provide means for rewriting a SELECT statement so that columns can be replaced with arbitrary data.

In 2.1 we need to be able to obfuscate columns. Rather than rewrite the result set in MaxScale, it is better to rewrite the query so that for instance.

select a, b from tbl;

is rewritten into

select 'abcd' as a, b from tbl;

before sending the query to a backend.