utils: Use std::string as keys in nonstatic_class_registry
utils: Use std::string as keys in nonstatic_class_registry

The sstring::compare functions was never updated to work with
std::string_view. We could fix that, but it seems better to just
switch to std::string.

With a working compare function we can avoid copying the argument
passed to to_qualified_class_name when an entry is found in the map.

Signed-off-by: Rafael Ávila de Espíndola <1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>

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