0.9.5.20:

0.9.5.20:
  * thread stacks are freed by the thread that exists next.
    There is at most one freeable stack at any time.

  Details:

  * protect_control_stack_*_page always operates on the current thread
  * the lisp thread object does not keep a pointer to the C thread struct
    because it can now be freed at any time
  * the interruption queue is moved to the lisp thread object
  * much simpler thread start/stop and gc interaction and locking
  * STATE_STARTING is removed