Use a modified size for transaction minimum fee calculation.

Summary:
Currently, we use the full transaction size for a minimum fee calculation.
This commit changes the behavior to charge for the additional (minimum) of
179 bytes it would take to spend any newly created outputs.

Test Plan:

make check && ./test/functional/test_runner.py

Reviewers: #bitcoin_abc, jasonbcox, deadalnix

Reviewed By: #bitcoin_abc, jasonbcox, deadalnix

Subscribers: deadalnix, jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1378