Use sl4j string parameterization in logging

I'm not sure if it was decided to not use the SLF4J string parameterization but i couldn't find anything about it :). Building the logging messages this way is [slightly](http://www.slf4j.org/faq.html#logging_performance) more efficient than the way it's done currently.

Is this something we could merge? Sorry if it became a bit too big...
The `+ ')'` is not necessary here anymore.  Also this `LOG.debug()` call is inside a check on `LOG.isDebugEnabled()`, so there is no need to use the `{}` notation here.