Data\Db: rename Query and Connection to Db...

Class names in namespaces should not be chosen as once we didn't have
such. The fact that we already did "use Db\Connection as DbConnection"
is the best hint that naming was wrong.

So this patch renames Db\Connection to Db\DbConnection and does the
same with DbQuery. DbQuery has been adjusted to fit our new SimpleQuery
and to handle the new Filter implementation.
Data\PivotTable: should work with new queries

There is still some work to do be done here. And we should/could
rethink columns() VS getColumns().
Data\Db\DbQuery: rename also in monitoring module

refs #6418
Data\Db\DbQuery: forgot to rename once

refs #6418