neorados: assoc_delete() uses allocator_traits

neorados: assoc_delete() uses allocator_traits

the std::allocator<T> member functions destroy() and deallocate() were
deprecated in c++17 and removed in c++20. call the static functions on
std::allocator_traits<T> instead

resolves the c++20 compilation error with clang13:

In file included from ceph/src/test/cls_fifo/bench_cls_fifo.cc:38:
ceph/src/neorados/cls/fifo.h:684:7: error: no member named 'destroy' in 'std::allocator<neorados::cls::fifo::detail::JournalProcessor<ceph::async::allocator_binder<boost::asio::executor_binder<(lambda at ceph/src/neorados/cls/fifo.h:1012:4), boost::asio::executor>, std::allocator<void>>>>'
    a.destroy(t);
    ~ ^
ceph/src/neorados/cls/fifo.h:1728:11: note: in instantiation of function template specialization 'neorados::cls::fifo::FIFO::assoc_delete<ceph::async::allocator_binder<boost::asio::executor_binder<(lambda at ceph/src/neorados/cls/fifo.h:1012:4), boost::asio::executor>, std::allocator<void>>, neorados::cls::fifo::detail::JournalProcessor<ceph::async::allocator_binder<boost::asio::executor_binder<(lambda at ceph/src/neorados/cls/fifo.h:1012:4), boost::asio::executor>, std::allocator<void>>>>' requested here
    FIFO::assoc_delete(h, this);
          ^
ceph/src/neorados/cls/fifo.h:1605:6: note: in instantiation of member function 'neorados::cls::fifo::detail::JournalProcessor<ceph::async::allocator_binder<boost::asio::executor_binder<(lambda at ceph/src/neorados/cls/fifo.h:1012:4), boost::asio::executor>, std::allocator<void>>>::handle' requested here
            handle(errc::inconsistency);
            ^
ceph/src/neorados/cls/fifo.h:857:8: note: in instantiation of member function 'neorados::cls::fifo::detail::JournalProcessor<ceph::async::allocator_binder<boost::asio::executor_binder<(lambda at ceph/src/neorados/cls/fifo.h:1012:4), boost::asio::executor>, std::allocator<void>>>::process' requested here
    p->process();
       ^
/usr/include/boost/asio/bind_executor.hpp:407:12: note: in instantiation of member function 'neorados::cls::fifo::FIFO::NewPartPreparer<ceph::async::allocator_binder<boost::asio::executor_binder<(lambda at ceph/src/neorados/cls/fifo.h:1012:4), boost::asio::executor>, std::allocator<void>>>::operator()' requested here
    return this->target_(BOOST_ASIO_MOVE_CAST(Args)(args)...);
           ^
ceph/src/common/async/bind_allocator.h:179:12: note: in instantiation of function template specialization 'boost::asio::executor_binder<neorados::cls::fifo::FIFO::NewPartPreparer<ceph::async::allocator_binder<boost::asio::executor_binder<(lambda at ceph/src/neorados/cls/fifo.h:1012:4), boost::asio::executor>, std::allocator<void>>>, boost::asio::executor>::operator()<boost::system::error_code &, bool>' requested here
    return this->target(std::forward<Args>(args)...);
           ^
ceph/src/neorados/cls/fifo.h:939:5: note: in instantiation of function template specialization 'neorados::cls::fifo::FIFO::_update_meta<ceph::async::allocator_binder<boost::asio::executor_binder<neorados::cls::fifo::FIFO::NewPartPreparer<ceph::async::allocator_binder<boost::asio::executor_binder<(lambda at ceph/src/neorados/cls/fifo.h:1012:4), boost::asio::executor>, std::allocator<void>>>, boost::asio::executor>, std::allocator<void>>>' requested here
    _update_meta(fifo::update{}.journal_entries_add(jentries),
    ^
ceph/src/neorados/cls/fifo.h:1008:7: note: in instantiation of function template specialization 'neorados::cls::fifo::FIFO::_prepare_new_part<ceph::async::allocator_binder<boost::asio::executor_binder<(lambda at ceph/src/neorados/cls/fifo.h:1012:4), boost::asio::executor>, std::allocator<void>>>' requested here
      _prepare_new_part(
      ^
ceph/src/neorados/cls/fifo.h:524:7: note: in instantiation of function template specialization 'neorados::cls::fifo::FIFO::_prepare_new_head<ceph::async::allocator_binder<boost::asio::executor_binder<neorados::cls::fifo::FIFO::Pusher<spawn::detail::coro_handler<boost::asio::executor_binder<void (*)(), boost::asio::executor>, void>>, boost::asio::executor>, std::allocator<void>>>' requested here
      _prepare_new_head(std::move(p));
      ^

Signed-off-by: Casey Bodley <cbodley@redhat.com>