fix mpsc DynamicBoundedQueue try_enqueue could fail even if the queue is empty

[wsu13]

Problem:

We used folly::DMPSCQueue<> q(1024) and encountered try_enqueue() return false frequently even if the queue is empty.

Debug:

I logged the three atomic value debit, credit, transfer before and after try_enqueue() operation, and saw the issue.

01-14   20:45:07.562839 INFO    17569   ***.cc:111 trace_id:***548   Submit task to worker1_12 Ok debit=1124, credit=2, transfer=1122, qsize=0, after push debit=1125, credit=2, transfer=1122
01-14   20:45:07.562888 INFO    17576   ***.cc:111 trace_id:***055   Submit task to worker1_12 Ok debit=1125, credit=3, transfer=1122, qsize=0, after push debit=1126, credit=3, transfer=1122
01-14   20:45:07.568038 INFO    17572   ***.cc:111 trace_id:***618   Submit task to worker1_12 Ok debit=1126, credit=4, transfer=1122, qsize=0, after push debit=7, credit=5, transfer=0
01-14   20:45:07.568161 INFO    17570   ***.cc:111 trace_id:***736   Submit task to worker1_12 Fail debit=1126, credit=4, transfer=1122, qsize=0, after push debit=7, credit=5, transfer=0

We use version v2018.08.20.00 the threshold_ is not rounded up, so the capacity is 1024+102 =1126.

  bool tryReduceDebit() noexcept {
    Weight w = takeTransfer();
    if (w > 0) {
      subDebit(w);
    }
    return w > 0;
  }

  Weight takeTransfer() noexcept {
    Weight w = getTransfer();
    if (w > 0) {
      w = transfer_.exchange(0, std::memory_order_acq_rel);
    }
    return w;
  }

At this case, capacity=1126, debit=1126, credit=4, transfer=1122, so the queue is actually empty.

Now there is 2 concurrent producers both do takeTransfer().
Producer_1 succeeded take transfer value, but haven't subDebit(w).
producer_2 saw transfer value is 0, and return, and check debit + weight > capacity, then try_enqueue() failed.

The unit_test in the pr can reproduce the case.

Fix:

Let prodcuer do tryReduceDebit() earlier, when debit reaches half capacity.

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