Restore CWallet::minTxFee

Summary:
This separtes config from the wallet and the rest of the node.

Depends on D3352

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3353