[fix](be) explicitly close stream load recorder RocksDB on shutdown

[sollhui]

What problem does this PR solve?

Problem Summary:

Explicitly close the RocksDB instance used by StreamLoadRecorder during destruction.

Previously, StreamLoadRecorder::~StreamLoadRecorder() only destroyed column family handles and synced WAL, then relied on DBWithTTL destruction to close RocksDB implicitly. If BE failed during startup, for example because be_port was already occupied, shutdown cleanup could enter RocksDB DBImpl::CloseHelper() from the implicit destructor path and crash while unscheduling background work.

This patch makes the shutdown sequence explicit:

• Sync WAL before closing
• Cancel RocksDB background work and wait for completion
• Destroy column family handles
• Close the DB explicitly
• Reset the DB pointer after close

Release note

None

Check List (For Author)

• Test

  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
  • No need to test or manual test. Explain why:
    • This is a refactor/code format and no logic has been changed.
    • Previous test can cover this change.
    • No code files have been changed.
    • Other reason

• Behavior changed:

  • No.
  • Yes.

• Does this need documentation?

  • No.
  • Yes.

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label

[hello-stephen]

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