txorphanage: make m_peer_work_set private

Summary:
Partial backport of core#26295:
https://github.com/bitcoin/bitcoin/pull/26295/commits/a936f41a5d5f7bb97425f82ec64dfae62e840a56

Depends on D16232.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16233