(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.
I thought about making logging accept a function, and only executing it if logging is enabled. That'd require rewriting most calls as lambdas. I'll take another look at how is_log_enabled is used; I'd rather not have it check a global.