re-visit external epoll

Hi:
I know I have discussed this with you before. However, I was busy integrating openssl 1.1.0b into our product with QAT. I have not had time to address this issue I ran into.
To start with, the new ASYNC API works well and it really make my life earlier and I do see improvement in term of performance.
yes, I have enabled EXTERNAL polling using openssl.cfg and that works. The issue now is the response event send from driver to epoll handler is not reliable. When epoll is running internally, I use Ixia to generate traffic and it can sustain heavy traffic for long period of time. When in external mode, it failed during handshaking with just few connections. As far as I can tell, the only difference is one start the callback thread during qat init. In my case, I start callback thread after qat init.

Please advice
Tseng