Make intrusive_set_external_comparator::_value_traits static
Make intrusive_set_external_comparator::_value_traits static

_value_traits can be shared among all instances
and there's no need to store it in every single one.

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

scylladb/scylla,raphaelsc/scylla,duarten/scylla,raphaelsc/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla,raphaelsc/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla