Separate Leaders and Followers into different raft workers.

[coocood]

Currently only a single raft worker process raft messages, the workload for leaders and followers are very different.

Leaders takes more time to handle messages, Append messages are sent before data persistence.

Followers takes less time to handle messages, AppendResponse messages are sent after data persistence.

If we separate leaders and followers, they don't need to wait for each other to finish, the latency would be lower, the resource utilization would be higher.

[coocood]

If a peer role changed after processed a raft message, we wrap the peer to a message and send it to the other worker.
If a message is sent to a worker of a different role, we forward the message to the other worker.

[coocood]

@hslam