Reorganize code and change behavior of ast_taskprocessor_execute() when taskprocessor is shutting down.

Moved code around to be easier to follow.

ast_taskprocessor_execute() will now return 0 if the taskprocessor is being shut down.



git-svn-id: https://origsvn.digium.com/svn/asterisk/team/mmichelson/threadpool@376499 65c4cc65-6c06-0410-ace0-fbb531ad65f3