alternator: defuse a serialization path time bomb
alternator: defuse a serialization path time bomb

The default serialization path for items was subtly broken -
instead of parsing JSON string representation of objects,
it tried to parse a regular string implementation - which is often
also a valid JSON, but nothing guarantees that it actually is.

Tests: alternator-test(local)

Message-Id: <332b6e699fdcc51e0889ce4791f2de72a323861b@scylladb.com>

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