Migrate/adapt signed_entity table

[jpraynaud]

Issue

Create a signed entity store and a new signed_entity table

Methodology

• Prepare migration script (data structure creation + data migration) + (destruction previous store, maybe later)
• Update the existing store (adapter modification only) if it is possible
• Create a new store otherwise
• Adapt/refactor runtime and/or http server to use the new store

Parent issue

#779