bgpd: Convert `struct peer_connection` to dynamically allocated

bgpd: Convert `struct peer_connection` to dynamically allocated

As part of the conversion to a `struct peer_connection` it will
be desirable to have 2 pointers one for when we open a connection
and one for when we receive a connection.  Start this actual
conversion over to this in `struct peer`.  If this sounds confusing
take a look at the bgp state machine for connections and how
it resolves the processing of this router opening -vs- this
router receiving an open.  At some point in time the state
machine decides that we are keeping one of the two connections.

Future commits will allow us to untangle the peer/doppelganger
duality with this abstraction.

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