vinyl: implement quota wait queue without fiber_cond

Using fiber_cond as a wait queue isn't very convenient, because:
 - It doesn't allow us to put a spuriously woken up fiber back to the
   same position in the queue where it was, thus violating fairness.
 - It doesn't allow us to check whether we actually need to wake up a
   fiber or it will have to go back to sleep anyway as it needs more
   memory than currently available.
 - It doesn't allow us to implement a multi-queue approach where fibers
   that have different priorities are put to different queues.

So let's rewrite the wait queue with plain rlist and fiber_yield.

Needed for #1862