(CFACT-66) Remove log4cxx

Replaced log4cxx with Boost.Log. Supports a similar structure of configuring formatted
sinks for output, with filtering based on log_level.
There's no measurable performance cost to this, so I'd rather avoid the complication of a global.

The slg.open_record is exactly the same check done in log, so this seems like the most maintainable check. I'll add a comment documenting that.