Non blocking artifact production in aggregator

[jpraynaud]

Why

We need to avoid blocking the signature of new open messages when the aggregator is computing the artifact for a signed entity type (an operation which can take up to several hours). In particular, this will help have a consistent certification of the Cardano transactions which is occurring at a faster pace.

What

Update the artifact production so that:

• The signed entity type is locked during the operation
• The signed entity type is unlocked when the operation terminates (success or failure)
• The computation of the artifact is done in a background thread

How

• Update the SignedEntityService create_artifact function:
  • Make the computation in a separate task
  • Return the task instead of unit
  • Lock signed entity type
  • Unlock signed entity type (when success or failure)
• Adapt the integration tests (if any problem occurs because of the background task)