Pull request description:
This is purely a refactor with no behavior changes.
This creates a new class `BCLog::Logger` to encapsulate all global logging configuration and state.
Tree-SHA512: b34811f54a53b7375d7b6f84925453c6f2419d21179379ee28b3843d0f4ff8e22020de84a5e783453ea927e9074e32de8ecd05a6fa50d7bb05502001aaed8e53
Partial backport of Core PR12954
https://github.com/bitcoin/bitcoin/pull/12954/files#diff-d278fc7d6997679f1f7416c5eda758fcR92
https://github.com/bitcoin/bitcoin/pull/12954/files#diff-d278fc7d6997679f1f7416c5eda758fcR147