[INC-660] Validate nested schemas in debug mode

[jjbayer]

In INC-660, a Kafka message with a valid msgpack schema contained an empty JSON object ("{}") as payload, which made the ingest consumer crash.

The existing validator on our Kafka producer, which only runs in debug mode, could catch bugs of this type (in local testing or integration tests) if it also validated the serialized payload in the Kafka message.