Class mClockQueue calls PullPriorityQueue::pull_request in function
"dequeue()" when OSD dequeues a request from opqueue. But
PullPriorityQueue::pull_request may return three types of values
namely "returning", "future", and "none". The type "future" means
that there are some requests in queue, but their tag are greater than
now, so they should be execute in the future but not now.
mClockQueue::dequeue() do not consider about the type of return value
and execute the request immediately. PullPriorityQueue only pops the
request when return "returning" request, so the "future" request would
be remained. This will cause the operation be executed in many times.
From Weihaocheng
