main_q is not used anywhere in the codebase.
It is created in TaskQueueManager._initialize_processes, bundled with rslt_q
into TaskQueueManger._workers, later unwrapped in StrategyBase but not used.
This queue is closed in TaskQueueManger._cleanup_processes.
Historically, it is passed as a init parameter into WorkerProcess,
introduced in 62d7956, but this behavior is changed in 120b9a7.
Signed-off-by: 夏恺(Xia Kai) <xiaket@gmail.com>