[Part 5] Refactor logging code into a global object

Summary: Cleanup. Remove dependencies on utilstrencodings and boost/mutex.

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

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

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Subscribers: deadalnix, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1284