Add back 'snapshot' feature

[luanzhilab]

[luanzhilab]

Introduce Learner role

Pending questions:

• 1. Leader election failed when Learner joined
• 2. Snapshot transmission during leader switching

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1. Leader election failed when Learner joined

When a new learner joins and the cluster is in a leaderless state (election not completed), according to the Raft protocol:

• The learner's AddLearner configuration change request will not be submitted (the leader needs to submit a log entry)
• The learner will continue to try to find the leader (through the election timeout mechanism)

2. Snapshot transmission during leader switching

Risks:

• The new leader may have a different log view
• Unfinished snapshots may contain outdated data