Fix http server lock while snapshotting in Mithril Aggregator

[jpraynaud]

The Mithril Aggregator http server does not respond when the runtime is creating a snapshot:

• Maybe a RwLock is in cause
• Maybe a green thread of tokio is in cause

[jpraynaud]

It seems that the problem is not related to a RwLock, curl -vvv http://aggregator.api.mithril.network/aggregator/123:

• when no snapshot/digest in computation returns a 404
• when snapshot/digest in computation does not return and is locked