KVStore: Fix unreleased Region instance

[CalvinNeo]

What problem does this PR solve?

Issue Number: close #9722

Problem Summary:

After a region is split, the new regions are stored in raft_cmd_res whose lifetime equals to KVStore. This struct will never be overwritten unless a new region is split.

As a result, the region can not be removed, even it is destroyed.

In this PR, we get rid of the raft_cmd_res, and we can found Regions can be destructed after it is destroyed.
We also added some metrics for further diagnose.

What is changed and how it works?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

None

[CalvinNeo]

/retest

[JaySon-Huang]

LGTM

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JaySon-Huang, Lloyd-Pottiger

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-01-06 07:23:59.532313209 +0000 UTC m=+165582.821144914: ☑️ agreed by Lloyd-Pottiger.
• 2025-01-07 03:43:40.927040728 +0000 UTC m=+238764.215872432: ☑️ agreed by JaySon-Huang.

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