feat: Modify to have only one RaftNode per physical node

[jopemachine]

Checklist: (if applicable)

• Milestone metadata specifying the target backport version
• Mention to the original issue
• Installer updates including:
  • Fixtures for db schema changes
  • New mandatory config options
• Update of end-to-end CLI integration tests in ai.backend.test
• API server-client counterparts (e.g., manager API -> client SDK)
• Test case(s) to:
  • Demonstrate the difference of before/after
  • Demonstrate the flow of abstract/conceptual models with a concrete implementation
• Documentation
  • Contents in the docs directory
  • docstrings in public interfaces and type annotations

[jopemachine]

• feat: Modify to have only one RaftNode per physical node #2104 👈
• feat/introduce-raftify

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[jopemachine]

Closing this PR as it was mistakenly created while introducing Graphite.
See #2108.