utils::loading_cache::on_timer(): use chunked_vector for storing elements we want to reload
utils::loading_cache::on_timer(): use chunked_vector for storing elements we want to reload

The list of elements that needs to be reloaded may be rather large.
Use chunked_vector in order to make the allocator's life easier.

Signed-off-by: Vlad Zolotarov <976c0f8d0852ed6439ad273880b400a75a7c41d0@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla