Simplify threadpool refcounting a bit.

Since threadpool shutdown is very strictly controlled,
there is no need to be so precise with reference counts
in queued operations. Since the threadpool shuts down its
own control taskprocessor before doing anything else destructive,
it can be guaranteed that all queued tasks will have a valid
pointer to the pool. This meant that some destructor functions
for helper structs could be removed entirely.



git-svn-id: https://origsvn.digium.com/svn/asterisk/team/mmichelson/threadpool@377210 65c4cc65-6c06-0410-ace0-fbb531ad65f3