core/slab: Destroy slab classes before destroying slab pages
core/slab: Destroy slab classes before destroying slab pages

slab classes contain free lists of slab pages. Destroying slab pages
which are still in the free lists will cause abort:

  memcached: /usr/include/boost/intrusive/detail/generic_hook.hpp:48: void boost::intrusive::detail::destructor_impl(Hook&, boost::intrusive::detail::link_dispatch<(boost::intrusive::link_mode_type)1>) [with Hook = boost::intrusive::generic_hook<(boost::intrusive::algo_types)0, boost::intrusive::list_node_traits<void*>, boost::intrusive::member_tag, (boost::intrusive::link_mode_type)1, (boost::intrusive::base_hook_type)0>]: Assertion `!hook.is_linked()' failed.

We should thus destroy slab classes first.

scylladb/seastar,avikivity/seastar,dreamsxin/seastar,cloudius-systems/seastar,scylladb/seastar,avikivity/seastar,avikivity/seastar,cloudius-systems/seastar,syuu1228/seastar,dreamsxin/seastar,dreamsxin/seastar,syuu1228/seastar,cloudius-systems/seastar,syuu1228/seastar,scylladb/seastar