bgpd: remove calls to peer_sort() from fast-path

bgpd: remove calls to peer_sort() from fast-path

  peer_sort() it's called so much as to be annoying. In the assumption
that the 'sort' of the peer doesn't change during an established session,
I have changed all calls to peer_sort() in the 'fast-path' to only check
the 'sort'. All the calls from the vty and such still recalculate the sort
and store it in the peer.

  There's a lot of other calls to peer_sort() that could be changed but some
maube tricky, someone more knowledgeable may try to reduce them.

  This hits peer_sort() from 5th out of the stadium^H^H list on a full
internet table loading profiling session.

Signed-off-by: Jorge Boncompte [DTI2] <jorge@dti2.net>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>