Simplify logs on the Signer/Aggregator

[jpraynaud]

Issue

Make the logs of signer and aggregator more clear and less verbose.

To do

• Implement manually debug for succinct logs of types with hex encoded keys
• Do we still display in DEBUG long lists (stake distribution, list of signers, ...)?
• Do we put these verbose logs in TRACE?

To explore / Later?

• Check if/how we can Implement context for logs (epoch in state machine, party_id involved on HTTP calls, ...)?
• Rewording of some error messages for clarity?