Fixed a shutdown problem with AsyncAppender in non-blocking mode

Hi,

I used an AsyncAppender in a Karaf pax-logging environment in non-blocking mode. As long as the AsyncAppender was active in non-blocking mode, the Karaf could not shutdown with CTRL+D, instead you had to shutdown with CTRL+C. 

In some investigation, I found out, that the Dispatcher thread was the problem, since there seems to be a deadlock situation in the OSGi shutdown procedure when using an AsyncAppender in non-blocking mode. However, with this patch the problem does not occur any more, but it has the downfall, that some messages are potentially lost in this way.

I hope someone gives me some oppinion on this change :-)

Kind regards,
Felix