Leader fault detection and rotation within primary execution committee

[ryscheng]

Any worker can act maliciously in an execution committee. The most problematic is the leader, who drives 1. transaction ordering/batching, 2. forwarding to other workers 3. coordinating commit-reveal, 4. storing states to storage.

Any of these steps can be mishandled. We need ways to

• Detect faulty leaders
• Prove the consensus layer of its faults
• Rotate leaders to satisfy liveness

[kostko]

Leader rotation depends on #366.