Cardano transactions import blocks aggregator and signer

[jpraynaud]

Why

The database insertions of the Cardano transactions are blocking the aggregator and signer nodes: the state machine stops operating and the REST API is down.

After investigation it appears that the database operations are sync and thus block the async executor.

What

Investigate and fix the blocking of the nodes during the import of the Cardano transactions.

How

• Investigate the origin of the blocking
• Fix the blocking