Convert rpc package to use Amount class
Summary: Converts RPC package to use Amount class.
Test Plan: make VERBOSE=1 check && ./qa/pull-tester/rpc-tests.py
Reviewers: freetrader, deadalnix, #bitcoin_abc
Reviewed By: freetrader, deadalnix, #bitcoin_abc
Differential Revision: https://reviews.bitcoinabc.org/D636