Decentralization of signature orchestration

[rezabaram]

Why

What

• Develop a signature orchestration mechanism which is compatible with a decentralized Mithril network.
• The implementation will work both with the current centralized setup for production, and with an experimental decentralized setup.

Acceptance Criteria

• The Mithril signer and aggregator nodes are able to compute independently the beacon which determines the messages to sign and certify.
• The Mithril aggregator does not advertise the pending certificates anymore
• The Mithril aggregator buffers the individual signatures from signers until it has computed the associated beacon, and it will try to aggregate them thereafter.