Fix that CWallet::AbandonTransaction would only traverse one level

Summary:
Prior to this change, it would mark only the first layer of
child transactions abandoned, due to always following the input hashTx
rather than the current now tx.

Backport of Core PR13652

Test Plan:

make check && ./test/functional/test_runner.py

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1978