utils_db_query: Introduced the query & result preparation areas.

utils_db_query: Introduced the query & result preparation areas.

These objects are used to store the temporary data needed during execution of
a query, which was previously stored in the query object itself. This allows
the query object to be used read-only, making it possible to use it from
multiple threads in parallel.

The dbi, oracle and postgresql plugins have been updated to use that
(mandatory) new feature.