Withdrawls and announcements attempt 2

Keeping track of the state of who got announced what is likely
to be a very very very bad idea for busy BGP talkers carrying
today's amount of NLRI and instability.

There are some hacks around the simplistic "if it's in my RIB,
I have to propagate withdrawals to all my neighbours" for some
cases, but a more comprehensive fix would require some Thinking.

This should probably get migrated over to the BGP list.