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