tpool: clear up some references that prevent garbage collection

https://github.com/eventlet/eventlet/pull/33
doc: hubs: Point to the correct function in exception message
green: subprocess: Popen.wait accepts new `timeout` kwarg

As in Python 3.3 http://docs.python.org/3.3/library/subprocess.html#subprocess.Popen.wait
https://bitbucket.org/eventlet/eventlet/issue/89/add-a-timeout-argument-to-subprocesspopen
https://bitbucket.org/eventlet/eventlet/pull-request/30