Nest: Fix race condition during reconfiguration, part 2

If export filter is changed during reconfiguration and a route disappears
between reconfiguration and refeed (e.g., if the route is a static route
also removed during the reconfiguration), the route is not withdrawn.

The issue was fixed for regular channels by an earlier patch. This patch
fixes the issue for channels in RA_ACCEPTED mode (first-pass-the-filter),
used by BGP with 'secondary' option.