[LOGMGR-107] Implement LogManager.getLoggerNames()

The J.U.L. ```LogManager.getLoggerNames()``` only returns loggers that have been created. Adding the ```boolean``` to the ```LoggerNode``` allows the ```org.jboss.logmanager.LogManager.getLoggerNames()``` to have the same results.

While in J.U.L the ```LogManager.getLoggingMXBean().getLoggerNames()``` does _not_ return loggers that have been created, our implementation has always done this. Therefore I decided to leave our implementation as is for now.
This was used in case a logger had been GC'd (see https://github.com/jboss-logging/jboss-logmanager/pull/57#discussion-diff-13261258). It just increases and decreases the counter.

This is a note form the JavaDoc:
>Note: Loggers may be added dynamically as new classes are loaded. This method only reports on the loggers that are currently registered. It is also important to note that this method only returns the name of a Logger, not a strong reference to the Logger itself. The returned String does nothing to prevent the Logger from being garbage collected. In particular, if the returned name is passed to LogManager.getLogger(), then the caller must check the return value from LogManager.getLogger() for null to properly handle the case where the Logger has been garbage collected in the time since its name was returned by this method.

It seems the name should be returned even if the logger was GC'd.