lock-free queue with RCU-based garbage collection

lock-free queue with RCU-based garbage collection

A lock-free queue example using RCU to avoid the need for double-word
compare-and-swap and, at the same time, to implement efficient garbage
collection.  I tested it lightly, and I don't think it's fully ready
for inclusion.  It does need the previous two patches to avoid deadlocks,
so I did test it somewhat. :-)

See individual files for detailed comments on how it works.

Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>