bgpd: Fix several use after free's in bgp for the peer

bgpd: Fix several use after free's in bgp for the peer

Three fixes:

a) When calling bgp_fsm_change_status with `Deleted` do
not add a new event to the peer's t_event because
we are already in the process of deleting everything

b) When bgp_stop decides to delete a peer return a notification
that it is happening to bgp_event_update so that it does not
set the peer state back to idle or do other processing.

c) bgp_event_update can cause a peer deletion, because
the peer can be deleted in the fsm function but the peer
data structure is still pointed to and used after words.
So lock the peer before entering and prevent a use after
free.

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