Convert wallet GetCredit function to use Amount class

Summary:
Convert wallet GetCredit funciton to use Amount class,
continuing T104

Test Plan: make check && ./qa/pull-tester/rpc-tests.py

Reviewers: deadalnix, freetrader, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D644