[wallet] Move maxTxFee to wallet

Summary:
This commit moves the maxtxfee setting to the wallet. There is only
one minor behavior change:

• an error message in feebumper now refers to -maxtxfee instead of

maxTxFee.

Backport of Core PR15778

Depends on D6266

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6267