net_processing: Localise orphan_work_set handling to ProcessOrphanTx

Summary:
Partial backport of core#26295:
https://github.com/bitcoin/bitcoin/pull/26295/commits/6f8e442ba61378489a6e2e2ab5bcfbccca1a29ec

Depends on D16230.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16231