bgpd: Do a bit better job of tracking the bgp->peerhash

bgpd: Do a bit better job of tracking the bgp->peerhash

When we add/remove peers we need to do a bit better job
of tracking them in the bgp->peerhash.

1) When we have the doppelganger take over, make sure the
winner is the one represented in the peerhash.

2) When creating the doppelganger, leave the current one
in place instead of blindly replacing it.

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