This can be used to re-feed routes to protocol after soft change in
export filters.

For transparent pipes, loop detection works correctly now.
Pipes are now more symmetric - in both directions filtering
is done in do_rte_announce().

If protocol announces a route, route is accepted by import filter to
routing table, and later it announces replacement of that route that is
rejected by import filter, old route remains in routing table.

That makes it easier to integrate BIRD to crosscompiling
buildsystems.

There is no reak callback scheduler and previous behavior causes
bad things during hard congestion (like BGP hold timeouts).

Smart callback scheduler is still missing, but main loop was
changed such that it first processes all tx callbacks (which
are fast enough) (but max 4* per socket) + rx callbacks for CLI,
and in the second phase it processes one rx callback per
socket up to four sockets (as rx callback can be slow when
there are too many protocols, because route redistribution
is done synchronously inside rx callback). If there is event
callback ready, second phase is skipped in 90% of iterations
(to speed up CLI during congestion).

Filters should try to copy-on-write just cached rtas.