Adaptive spinning in write path with readers and writer starvation avoidance.

 - Move recursion checking into rwlock inlines to free a bit for use with
   adaptive spinners.
 - Clear the RW_LOCK_WRITE_SPINNERS flag whenever the lock state changes
   causing write spinners to restart their loop.
 - Write spinners are limited by a count while readers hold the lock as
   there is no way to know for certain whether readers are running still.
 - In the read path block if there are write waiters or spinners to avoid
   starving writers.  Use a new per-thread count, td_rw_rlocks, to skip
   starvation avoidance if it might cause a deadlock.
 - Remove or change invalid assertions in turnstiles.

Reviewed by:    attilio (developed parts of the patch as well)
Sponsored by:   Nokia

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