Fixes/Optimizations Mithril Aggregator

[jpraynaud]

Some bug fixes and optimizations are needed for the Mithril Aggregator:

• Fix debug logs never accessible on release build (update release_max_level_warn to release_max_level_debug in Cargo.toml for log dependency)
• On Ubuntu, when we launch the Docker container, the server is not reachable
• A graceful shutdown implementation would help to quit easily a running container
• Move the openapi.yaml to mithril-network and update all the dependencies
• Add missing field in OpenAPI specification as explained in Add missing field in OpenAPI specification #112