[Bug] The leader election maybe not fair when there is a malicious node

[fCorleone]

🐛 Bug

The leader times of various nodes may be unfair.

To reproduce

I started a swarm with 10 nodes, 1 of them is a malicious one. And I then generate the transactions by using the command 'cluster-test'. The times for each node to be the leader varies among each other with up to a variance of 37.5%.

Expected Behavior

I believe DiemBFT tries to guarantee that the leader election process is fair among each nodes. And each node should be elected as a leader with the same frequency.

System information

Please complete the following information:
Diem: branch testnet
Rust version: cargo 1.61.0-nightly
Ubuntu 20.04

Additional context

The logs are attached here:
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