Surround calls to log() function with an if statement to avoid costly string concats if the logger is not enabled

This is an obviously low-priority RFE

Would be a nice idea to surround all calls to the logging code with
if(logger.isDebugEnabled(this)) or logger.isInfoEnabled() to avoid the cost of string concats in the case of loggers actually being filtered out.

this is probably only applicable to all the incoming/outgoing FIX message logging, may not be that expensive but probably a good idea.