bgpd: avoid notify race between io and main pthreads

bgpd: avoid notify race between io and main pthreads

The "bgp_notify_" apis in bgp_packet.c generate a notification
to a peer, usually during error handling. The io pthread wants
to send notifications in a couple of cases during early
received-packet validation - but the existing api interacts
with the peer struct itself, and that's not safe.

Add a new api for use by the io pthread, and adjust the main
notify api so that it can avoid touching the peer struct.

Signed-off-by: Mark Stapp <mstapp@nvidia.com>