vinyl: cache dump bandwidth for timer invocation

We don't need to compute a percentile of dump bandwidth histogram on
each invocation of quota timer callback, because it may only be updated
on dump completion. Let's cache it. Currently, it isn't that important,
because the timer period is set to 1 second. However, once we start
using the timer for throttling, we'll have to make it run more often and
so caching the dump bandwidth value will make sense.