Override base class close.
This method waits while there are queued work items. The events are sent asynchronously using ThreadPool work items. These items will be sent once a thread pool thread is available to send them, therefore it is possible to close the appender before all the queued events have been sent.
This method attempts to wait until all the queued events have been sent, but this method will timeout after 30 seconds regardless.
If the appender is being closed because the ProcessExit event has fired it may not be possible to send all the queued events. During process exit the runtime limits the time that a ProcessExit event handler is allowed to run for.