[backport#15644] Simplify orphan processing in preparation for interruptibility
[backport#15644] Simplify orphan processing in preparation for interruptibility

Summary:
https://github.com/bitcoin/bitcoin/pull/15644/commits/9453018fdc8f02d42832374bcf1d6e3a1df02281

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Partial backport of Core [[https://github.com/bitcoin/bitcoin/pull/15644 | PR15644]]

Test Plan:
  ninja check
  ./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, deadalnix, nakihito

Reviewed By: #bitcoin_abc, deadalnix, nakihito

Subscribers: nakihito, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6216

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