kernel - Carefully refactor contended tokens and spinlocks

kernel - Carefully refactor contended tokens and spinlocks

* Carefully put the exponential backoff back in for spinlocks, and
  implement for tokens.  Only applicable to exclusive locks, capped
  via sysctl (mjg).  Tested on dual-socket xeon.

* Exclusive priority for shared locks reduces the shared/exclusive
  starvation that can occur when exclusive locks use exponential
  backoff.

* Exponential backoff significantly improves performance for
  heavily contended exclusive locks by allowing some degree of burst
  operation.

* Implement TSC windowing for shared locks (and a little for
  exclusive locks too).  This prevents heavily contended exclusive
  locks from completely starving shared locks by using windowing
  to disable the exclusive-priority mechanic for shared locks.

  This allows a few contending shared locks to compete on equal
  ground with exclusive locks.

Suggested-by: mjg