lib: bring some changes over from bgp_table to table

lib: bring some changes over from bgp_table to table

  * lib/table.c

    - Maintain table node count. Expose it via the route_table_count()
      function (from revision cbdfbaa5).

    - route_unlock_node(): Add assertion (from revision 228da428).

    - route_table_free(): Make static and fix up cleanup code (from
      revision 228da428).

    - route_node_delete(): Change to be static.

    - Add 'const' qualifier in a couple places.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>