dbi plugin: Put the row handling logic in an extra module.

dbi plugin: Put the row handling logic in an extra module.

The `utils_db_query' module now handles config parsing and row handling.
This unifies basically all of the <Query> blocks, so that the `dbi' and
`oracle' plugins behave exactly alike. Porting the `postgresql' plugin
should be possible, too, so that all three database plugins are in line.