vinyl: don't consume quota if wait queue isn't empty

vy_quota_use only checks if there's enough quota available for consumer
to proceed, but that's not enough, because it may occur that there are
fibers already waiting for the resource. Bypassing them may result in
starvation, which manifests itself as "waited for vinyl memory quota for
too long" warnings. To ensure fairness and avoid starvation, let's go to
sleep if the wait queue is not empty.