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

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

Partial backport of Core 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