utils: Use sstring as keys in nonstatic_class_registry
utils: Use sstring as keys in nonstatic_class_registry

Now that seastar::string::compare has been updated, it is possible to
use sstring for this.

This reverts commit 01fe766f1f31ce42e9bdaea4e2be0a8d893c4945.

Signed-off-by: Rafael Ávila de Espíndola <1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>
Message-Id: <20200311005219.280737-1-1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>

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