Nest: Removing separate tmpa from route propagation

This is a fundamental change of an original (1999) concept of route
processing inside BIRD. During import/export, there was a temporary
ea_list created which was to be used instead of the another one inside
the route itself.

This led to some confusion, quirks, and strange filter code that handled
extended route attributes. Dropping it now.

The protocol interface has changed in an uniform way -- the
`struct ea_list *attrs` argument has been removed from store_tmp_attrs(),
import_control(), rt_notify() and get_route_info().