------------------
- allow threads to dynamically enter and exit rcu's token passing ring
- augment rcu with heartbeat manager to kill and recover from stalled threads
- make rcu try yielding when its buffer gets full
------------------
- allow threads to dynamically enter and exit rcu's token passing ring
- augment rcu with heartbeat manager to kill and recover from stalled threads
- make rcu try yielding when its buffer gets full