txorphanage: Extract AddChildrenToWorkSet

Summary:
Extract some common code into AddChildrenToWorkSet function.

(It's a hard knock life)

This is a backport of core#21148 [4/14]
https://github.com/bitcoin/bitcoin/pull/21148/commits/ee135c8d5b39b0cb8b301a83e286285ab926dca7

Depends on D11483

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Subscribers: sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D11484