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/from_db_value/models.py

@@ -18,7 +18,7 @@ class CashField(models.DecimalField):
         kwargs['decimal_places'] = 2
         super(CashField, self).__init__(**kwargs)
 
-    def from_db_value(self, value, connection):
+    def from_db_value(self, value, connection, context):
         cash = Cash(value)
         cash.vendor = connection.vendor
         return cash