wen_restart: Change HeaviestFork stage to a leader based approach.

[wen-coding]

The original all distributed HeaviestFork aggregation can lead to divergence when different validators sees different messages.

Change to a leader based approach to guarantee convergence:

• Only the leader sends out HeaviestFork message, and it keeps sending out HeaviestFork every 5 minutes until manually stopped
• All others only accept HeaviestFork from the leader. After getting HeaviestFork from the leader, they would do verification and then use the (slot, hash) specified by the leader.