Allow LearnLogger to accept format strings

[mtf90]

The LearnLogger currently extends the org.slf4.Logger interface, which gives access to several nice logging methods using format strings, i.e. we don't have to directly construct the strings being logged (potentially including toString()s on data objects).

This functionality should also be offered for the specific learning related log methods (logQuery(), etc.). Since this will add a lot of boilerplate code, we may look into using something like Project Lombok to generate either the interface or at least the implementation (thus replacing the current Slf4jDelegator)