Add new functionality to the pipeline interface:

• Allow checking whether the pipeline is idle with an idle() member function
  • Requires each worker thread to notify whether it's idling (waiting for input, or a paused producer)
  • May require separating the pop_unless(...) member function in wait_unless(...) and try_pop() in order to not degrade performance by updating idle state every time (e.g. false sharing)
• Allow blocking the main thread until completion, similar to TBB flow graph's wait_for_all
  • In order to do it while actually blocking, without timer-based wake-ups, use a "listener" class,