Genericize the allocation and destruction of taskprocessor listeners.

The goal of this is to take the responsibility away from individual
listeners to be sure to properly unref the taskprocessor.



git-svn-id: https://origsvn.digium.com/svn/asterisk/team/mmichelson/threadpool@376121 65c4cc65-6c06-0410-ace0-fbb531ad65f3