Move minRelayFee into Config handler

Summary:
This commit is general cleanup. It moves the minRelayFee
global into the global configuration handling object.

Depends on D1173

Test Plan: make check

Reviewers: #bitcoin_abc, jasonbcox, deadalnix

Reviewed By: #bitcoin_abc, jasonbcox, deadalnix

Subscribers: deadalnix, jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1361