Pipe leaks from HelperClient due to Python multiprocessing Queue bug

[weili-jiang]

Possibly due to https://bugs.python.org/issue27151, I find that the HelperClient queue is leaking pipes if I create and stop multiple HelperClients in succession.

Regardless of what happens with the multiprocessing queue issue in Python itself, there is no reason HelperClient needs to use a multiprocessing queue at all because it does not do multiprocessing - creating OS pipes is therefore wasteful. To synchronize under multi-threading, the standard synchronized queue class is sufficient.