(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.
Given that this is a hot path (called for every single logging macro we use), I think it's probably best to just keep track of the current log level (last one passed to configure logging, I suppose) and do a `level >= current_level` comparison.