Regression in b3db6c8dcb.
Thanks Ian Cubitt for the report.
This also corrected test_inheritance_deferred2() test which was
previously properly defined and marked as an expected failure but was
then wrongly adjusted to mask the lack of support for per-alias
deferral that was fixed by #21204.
Refactored compiler SELECT, GROUP BY and ORDER BY generation.
While there, also refactored select_related() implementation
(get_cached_row() and get_klass_info() are now gone!).
Made get_db_converters() method work on expressions instead of
internal_type. This allows the backend converters to target
specific expressions if need be.
Added query.context, this can be used to set per-query state.
Also changed the signature of database converters. They now accept
context as an argument.
