TNonblockingServer blocked int addTask(IOThread) and notify(workerThread)

hello!

when using c++ TNonblockingServer(with thread pool)，I found it blocked in high QPS status.

I used pstack to print thread stack and found worker thread and IO thread blocked at:

The worker thread will call notifyIOThread when it handle expired task, then call TNonblockingIOThread::notify and wait for POLLOUT in poll without timeout. 

The IO thread will call addTask when it gets requests. And IO threads will lock threadManager->mutex_ in addTask without a timeout

Is it a bug of thrift 0.12.0?