Snapshot Trigger Condition

[JoshuaChi]

1. Large Log Gap:
   When a follower’s log lags behind the leader’s by more than a certain threshold (e.g., 10,000 uncommitted entries in etcd by default), the leader will send a snapshot directly instead of attempting to replicate entries one by one.

2. Persistent State Check:
   The leader maintains lastIncludedIndex and lastIncludedTerm. If a follower’s prevLogIndex is less than the lastIncludedIndex, it indicates that the follower’s log has been compacted and must be brought up to date via snapshot installation.（When the leader detects that a follower’s next_index is lower than the last_purged_index, it should immediately send a snapshot.）

3. When new node joins Cluster as Learner