Idle interface

[JoelFilho]

First tentative for the pipeline idling interface. Closes #3.

Some questions need to be answered before merging:

• Is it possible to have false positives on wait_until_idle(), between removing an item internally on the queue, and exiting a worker thread's idle state? (TODO: either demonstrate with a test case or show why it can't)
• Does the O(1) lookup on the bitset makes it better than an array of atomic boolean? Is copy-and-swap much slower than basic false sharing? Is there a better lock-free alternative? (TODO: benchmark, analyze the possibility of race conditions and invalid states)
• This incurs an overhead every iteration on each thread. Should it be opt-in only? (TODO: benchmark)