feat: Delete Progress.PendingSnapshot and improve snapshot handling

[soma00333]

Description

This PR addresses #124 by removing the unused Progress.PendingSnapshot field and improving snapshot handling in the Raft library.

What does this PR do?

1. Removes Progress.PendingSnapshot

   • Deletes the unused PendingSnapshot field from the Progress struct
   • Simplifies state management and reduces code complexity
   • Removes all related code and comments

2. Improves ReportSnapshot() API

   • Adds appliedSnapshotIndex parameter to track the actual applied snapshot index
   • Updates both node.go and rawnode.go implementations
   • Fixes incorrect documentation that claimed SnapshotFinish was a no-op

3. Enhances MsgSnapStatus handling

   • Adds applied_snapshot_index field to the protobuf message
   • Implements OnSnapshotApplied() method for cleaner state updates
   • Improves state transition logic after snapshot application
   • Adds debug logging for better troubleshooting

4. Optimizes state transitions

   • Directly transitions to StateReplicate when follower is caught up
   • Avoids unnecessary StateProbe phase when possible

Why is this needed?

The PendingSnapshot field was not being used effectively and added unnecessary complexity to the codebase. This change:

• Simplifies the snapshot handling logic
• Provides more flexibility for systems like CockroachDB that generate snapshots differently
• Makes state transitions more predictable and efficient

Breaking Changes

⚠️ This is a breaking change targeted for v4.0.0

• ReportSnapshot() now requires an appliedSnapshotIndex parameter
• MsgSnapStatus message format has changed
• Applications using these APIs will need to be updated

Testing

• All existing tests pass
• Added new test case for backward compatibility (when AppliedSnapshotIndex is 0)
• Updated testdata files to reflect the changes

Related Issues

• Fixes Clean up and improve snapshot handling #124
• Related to Accept any snapshot that allows replication #84 and Accept any snapshot that allows replication  #110 (previous snapshot handling improvements)

[k8s-ci-robot]

Hi @soma00333. Thanks for your PR.

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[soma00333]

@ahrtr
Hello, I hope you are doing good.
I handled one of the tickets in the v4.0.0 milestone.
Please take a look after you prepare the branch
Thank you

[serathius]

cc @joshuazh-x
Can you help with review?

[soma00333]

Thank you for checking this
I appreciate your help

[soma00333]

/retest

[k8s-ci-robot]

@soma00333: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

In response to this:

/retest

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[k8s-ci-robot]

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This pull-request has been approved by: soma00333
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