OPENDJ-2494 Improve OpenDJ SLF4J implementation

PR-195
This commit improves the OpenDJ SLF4J logger implementation as follow:
* Add a generic LocalizableMessage to log external library messages (e.g
common audit, grizzly, JE).

* If external libraries calls methods that we use in OpenDJ we do not
throw exception any more, we just log the message.

* As third party libraries could use our slf4j implementation, add fast
path checks to ensure that log level is enabled before doing anything.

* Log third party readme debug messages into the debug logger
(instead of the error logger with INFORMATION severity before).