Convert policy fees.cpp/.h to use Amount class

Summary:
Convert fees.cpp and fees.h to use Amount class. Update
dependencies where appropriate. FeeStats could not be swapped
from using double due to a file format dependency.

Test Plan: make VERBOSE=1 check && ./qa/pull-tester/rpc-tests.py

Reviewers: deadalnix, freetrader, #bitcoin_abc

Reviewed By: freetrader, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D634