[Part 2] Refactor logging code into a global object

Summary:
util: Establish global logger object.

The object encapsulates logging configuration, and in a later commit,
set up routines will also be moved into the class.

Test Plan:
Run bitcoind with different permutations of:
-logtimestamps, -debug, -printtoconsole, -datadir, -shrinkdebugfile

Reviewers: jasonbcox, deadalnix, O1 Bitcoin ABC, #bitcoin_abc, schancel

Reviewed By: jasonbcox, deadalnix, O1 Bitcoin ABC, #bitcoin_abc, schancel

Subscribers: schancel, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1281