[sql] preprocess_entity uses lgdb helper's SQL converters.
The preprocess_entity does not yield anymore the conversion logic for different
base types, but relies on a new SQL_CONVERTERS dictionary of the db helper.
This will allow later inclusion of new base types.
doc/book usr/share/doc/cubicweb-documentation
doc/html usr/share/doc/cubicweb-documentation
debian/cubicweb-doc usr/share/doc-base/cubicweb-doc