Prepare several files for explicit Amount construction

Summary:
Prepare init.cpp, miner.cpp, net.cpp, net_processing.cpp
for explicit Amount constructor. Continuing T104.

Test Plan: make check

Reviewers: mbhinder, deadalnix, freetrader, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D677