Use bencode for serializing Kafka keys.
Unlike msgpack, bencoding is deterministic, which is needed for objects to have a unique key in Kafka.
This commit also removed kafka_to_key, because it's unused (keys can be rebuilt from a message's value.
Depends on swh-core!80 (closed)
Test Plan
wait for swh-core!80 (closed) to be merged and released, then tox
Migrated from D2026 (view on Phabricator)