Fixed #24020 -- Refactored SQL compiler to use expressions

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.

tests/aggregation/tests.py

@@ -679,7 +679,7 @@ class BaseAggregateTestCase(TestCase):
                 # the only "ORDER BY" clause present in the query.
                 self.assertEqual(
                     re.findall(r'order by (\w+)', qstr),
-                    [', '.join(forced_ordering).lower()]
+                    [', '.join(f[1][0] for f in forced_ordering).lower()]
                 )
             else:
                 self.assertNotIn('order by', qstr)