libtracker-miner: Ensure harder process_stop() is only called once
libtracker-miner: Ensure harder process_stop() is only called once

QUEUE_WAIT is now used more extensively, meaning item_queue_handlers_cb()
onky gets QUEUE_NONE if there really is nothing left to process, be it
items in the task pool, pending to be crawled, or in the processing queues.

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