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.