Add a shutdown callback to taskprocessor listeners.

This helps account for the fact that it is unknown just
how many references may exist for a given taskprocessor
listener, so simply unreffing it from the taskprocessor
shutdown function is not enough to convey the gravity
of the situation.

By putting in a shutdown callback, it now becomes clear
to the listener not to try to do any further operations
on the taskprocessor.



git-svn-id: https://origsvn.digium.com/svn/asterisk/team/mmichelson/threadpool@376381 65c4cc65-6c06-0410-ace0-fbb531ad65f3

include/asterisk/taskprocessor.h

@@ -86,6 +86,18 @@ struct ast_taskprocessor_listener_callbacks {
 	 * \param listener The listener
 	 */
 	void (*emptied)(struct ast_taskprocessor_listener *listener);
+	/*!
+	 * \brief Indicates the taskprocessor wishes to die.
+	 *
+	 * All operations on the task processor must to be stopped in
+	 * this callback.
+	 *
+	 * After this callback returns, it is NOT safe to operate on the
+	 * listener's reference to the taskprocessor.
+	 *
+	 * \param listener The listener
+	 */
+	void (*shutdown)(struct ast_taskprocessor_listener *listener);
 	/*! 
 	 * \brief Destroy the listener's private data
 	 *