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

This also fixes bug that timer->recurrent was not cleared
in tm_new() and unexpected recurrence of startup timer
in BGP confused state machine and caused crash.

Previous bugfix revealed another hidden bug here.

Although it is true unless there is a bug in BIRD,  this assert is not
needed (code below does not require that assumption), so we should not
crash.

When update changes preferred route in such a way that
it ceased to be preferred, the new route was linked
to wrong place and lost.

Previous version of Makefile executed Bison two times (in parallel),
because of two specified targets. I am not sure wheter this is the best
fix.

Previon

Also does some code restructuring.