hints: de-template scan_for_hints_dirs()
hints: de-template scan_for_hints_dirs()

This function is called twice, and is not doing anything performance critical,
so replace the template parameter Func with std::function<>.x

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