+ Concurrent queue with wait-free enqueue. Both blocking and
+ non-blocking dequeue, splice (move all elements from one queue
+ to another), and traversal operations are provided.
+ This queue does _not_ specifically rely on RCU. Mutual exclusion
+ is used to protect dequeue, splice (from source queue) and
+ traversal (see API for details).
+ (note: deprecates urcu/wfqueue.h)