wallet: Make fee settings non-static members

Summary: This isa backport of Core PR12909

Test Plan:

make check
test/functional/test_runner.py

Reviewers: #bitcoin_abc, Fabien, jasonbcox

Reviewed By: #bitcoin_abc, Fabien, jasonbcox

Subscribers: jasonbcox, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3364