lib/table: add route_table_get_next() and iterator

lib/table: add route_table_get_next() and iterator

  * lib/table.[ch]

    - Add a function (route_table_get_next()) to get the route_node in
      a tree that succeeds a given prefix in iteration order.

      This allows one to reliably walk nodes in a tree while allowing
      modifications, and is useful for achieving scale and
      performance. Other approaches are also possible -- the main plus
      point of this one is that it does not require any state about
      the walk to be maintained in the table data structures.

    - Add an iterator for walking the nodes in a tree. This introduces
      a new structure (route_table_iter_t) and the following main
      functions.

        route_table_iter_init()
        route_table_iter_pause()
        route_table_iter_next()
        route_table_iter_cleanup()

      The iterator normally uses node pointers and the existing
      route_next() function to walk nodes efficiently. When an
      iteration is 'paused' with route_table_iter_pause(), it stores
      the last prefix processed. The next call to
      route_table_iter_next() transparently invokes
      route_table_get_next() with the prefix to resume iteration.

  * bgpd/bgp_table.[ch]

    Add wrappers for the new table features described above.

  * tests/table_test.c

    Add tests for the new table code.

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