bgpd: use Jenkins hash for BGP transit, cluster and attr hashes

bgpd: use Jenkins hash for BGP transit, cluster and attr hashes

* bgp_attr.c: I observed while doing some debugging that even for simple
  tests there was a lot of hash collisions for BGP attributes.  Switch to
  using Jhash rather than additive hashing.  Probably overkill, but the
  function is fast and available.

  ({attrhash,cluster,transit}_hask_key_make) convert to Jenkins hash,
  instead of additive hash.