wallet: always do avoid partial spends if fees are within a specified range

Summary:

• wallet: try -avoidpartialspends mode and use its result if fees are below threshold

The threshold is defined by a new max avoid partial spends fee flag, which defaults to 0 (i.e. if fees are unchanged, use the grouped option).

• test: test the implicit avoid partial spends functionality

Co-authored-by: Fabian Jahr <fjahr@protonmail.com>

This is a backport of Core PR14582

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D7807