[backport#15931 3/9] Replace CWalletTx::SetConf by Confirmation initialization list
Summary:
https://github.com/bitcoin/bitcoin/pull/15931/commits/9700fcb47feca9d78e005b8d18b41148c8f6b25f
Depends on D7507
Partial backport of Core PR15931
Test Plan:
ninja check check-functional
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D7508