Relational schema: properly create HistoryEvent

[lutter]

When we create a HistoryEvent, we write a row into event_meta_data. For relational schemas, that entry is neither needed nor used. We need to make sure that for relational schemas any HistoryEvent we create does not cause an entry in event_meta_data.

[lutter]

This is tricky, because the subgraph of subgraphs still uses JSONB storage, and therefore any changes that involve metadata that needs to be versioned (like dynamic data sources) need an entry in event_meta_data.

We either have to make the decision whether to allocate in event_meta_data in transact_block_operations, or move all that logic entirely into the database and have a trigger on subgraphs.entities, though that risks collecting unnecessary history for metadata