[IOPRIO] request gets a beancounter
Previously, we could release a beancounter that has requests in-flight.
Now, every request gets a ub, so beancounter can disappear only after
all requests are accomplished. This reasoning concerns async requests
only. Also note, that now the final put of beancounter can occur with
queue lock held, so we modify release_beancounter() function in order it
always use another thread for actual releasing. Thanks to xemul@ for