QC: Provide more detailed field information when it is available.

Currently the query classifier returns the affected fields as a simple string, always without the table name and without distinguising whether a field is used in the select part or the where part.

For instance, the affected fields of a query like

select x.a, y.b from x join y where x.c = 3

is reported as "a b c".

Extend the API so that no information is filtered.