storage/database: cleanup old database after migration

[hyeonLewis]

Proposed changes

When the node stops while removing the old db, the state migration restarts from scratch. This PR sets the old db path before removal so the node can find a pending removal job when restarted. Inspired from #229 (comment)

Please note that PR #229 only fixes nil dereference issue.

The overall process will be:

1. Set the completion mark and old db path.
2. Start removing old db.
3. The node stops before completely removing the old db.
4. The node will find the old db path and remove it.
5. Set empty old db path.

Types of changes

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• Bugfix
• New feature or enhancement
• Others

Checklist

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Related issues

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Further comments

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[hyeonLewis]

PTAL @hyunsooda