Use String instead of JsonNode for Jsonb snapshot in Entities

[yuanchen233]

All snapshots in database are saved as Jsonb, there are currently two types in Kotlin that can be saved directly as Jsonb, String and jackson.databind.JsonNode. We should switch to String for snapshots in all entity class.

Reasons:

1. JsonNode will validate the data and ensure a valid json when being created, but we have core/kotlin serializer handles it.
2. JsonNode is slow, as each json key-value pair is saved as a node. The process time to create a JsonNode increases significantly as the size of snapshot increases.
3. JsonNode provides the passibility to work with the key-value pairs it contains. However, there are limited usage for this is webservices currently, and we should convert the snapshots into its corresponding domain/datatype before we work with them, or access them directly through database.