bgpd: Fix peer withdrawal and route leaking for vpn's and vrf's

bgpd: Fix peer withdrawal and route leaking for vpn's and vrf's

When a peer is removed the routes are withdrawn via bgp_process_main_one
As such we need to put a bit of code in to handle this situation
for the vpn/vrf route leaking code.

I think this code path is also called for when a vrf's route is
changed and I believe we will end up putting a bit more code
here to handle the nexthop changes.

I've also started trying to document the bgp_process_main_one
function a bit better.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>