token_metadata: Remove std::iterator from tokens_iterator
token_metadata: Remove std::iterator from tokens_iterator

std::iterator is deprecated since C++17 so define all the required
iterator_traits directly.

Signed-off-by: Piotr Jastrzebski <2c0eb4a25a7cde333ff2d0860cb06051925fd74f@scylladb.com>

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