Logging: Add conditional logging macros.

[pyjarrett]

Purpose

Extend the logging system to allow logging which doesn't trigger on every line hit.

Method

Extend the logging system to include conditional logging variations of the existing CN_ERROR, CN_WARN and CN_TRACE macros to include these variations:

• *_IF(condition, msg, ...) - only log if the given condition is met
• *_EVERY(num, msg, ...) - only log every num times this log line is encountered
• *_ONCE(msg, ...) - only log the first time this log line is encountered