Cleanup TaskQueueSuper<>::peek

The TaskQueueSuper<>::peek function simply returns true if the queue's top and bottom differ. This means it is not being careful with the underflow possible during concurrent pop_local and pop_global.

However, it appears to only be used in assertions to verify the queue is empty after a successful termination negotiation. In that case a temporary underflow can't be happening because there shouldn't be any pop_local or pop_global operations in progress.

So while the current implementation suffices, it would be better to have a debug-only operation with a name (and possibly behavior) more suited to its usage.