move db close out of gc so no more thread errors

[wjblanke]

DO NOT MERGE plz

---

Note

Removes implicit DB close from Mempool.__del__ and explicitly closes the mempool SQLite connection in MempoolManager.shut_down() and after slow-path new_peak reinit, with added logging.

• Mempool lifecycle:
  • Remove implicit DB close by deleting Mempool.__del__.
  • Explicitly close mempool DB connection:
    • On shutdown in MempoolManager.shut_down() (with info log).
    • After slow-path mempool rebuild in MempoolManager.new_peak() (old_pool._db_conn.close()).

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