refactor: msg pool to make more structured part 3

[akaladarshi]

Summary of changes

Changes introduced in this pull request:

This PR is part 3 of restructuring of msg pool, it contains:

• Use Arc on MessagePool itself rather than each individual field, this will allow us to:
  • Pass the message pool directly into the head-change trigger which will become part of the MessagePool, instead of being a free function with unlimited params
  • Convert republish_cycle part of the MessagePool, instead of being a free function with unlimited params
  • Make all the message pool related free fn part of the Message pool methods:
    • head_change -> apply_head_change
    • republish_pending_messages -> run_republish_cycle
    • add_helper -> add_to_pool_unchecked
    • add_tipset -> add_to_pool

Reference issue to close (if applicable)

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Other information and links

Change checklist

• I have performed a self-review of my own code,
• I have made corresponding changes to the documentation. All new code adheres to the team's documentation standards,
• I have added tests that prove my fix is effective or that my feature works (if possible),
• I have made sure the CHANGELOG is up-to-date. All user-facing changes should be reflected in this document.

Outside contributions

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• I have read and agree to the AI Policy document. I understand that failure to comply with the guidelines will lead to rejection of the pull request.

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