Prepare code to plug the overcommit notification.

Let the pwqr_sb have a state (instead of yes/no "dead" flag) being among:
- DEAD
- NONE (normal)
- UC (undercommit)
- OC (overcommit).

In the last two modes a timer is fired.

In the UC case, if the timer fires, we unpark a thread (if any, and if no
overcommit unpark is pending) as before.

In the OC case for now we do nothing.

Signed-off-by: Pierre Habouzit <pierre.habouzit@intersec.com>