Fix a bug introduced in refactoring from batfish#6251, where
some routes would not be exchanged on BGP links that come up after
the snapshot starts. Fairly rare in datacenters, because BGP links in
datacenters are usually ebgp singlehop.

When sending routes to new links, we need to use the main RIB from
the previous round rather than the current RIB, because depending on
iteration order of the current round the current RIB may include
routes that were not yet advertised in the previous round. This adds
some memory overhead, but pointers to the routes are much smaller
than the routes themselves, so this should be less than 10%.

Add a test.

* make invariants more clear and enforced in only one place
* not track the previous entire RIB unless necessary

The main thing missing was to defer processing of external
advertisements to the first round of BGP instead of doing it
in the neutral zone between IGP and EGP computations.