Changes:
- Support HTTP/1.1 persistent connections
- Add option 'O' for specifying timeout of persistent connections
- Add new counters kcc, kmx, kvg and krq related to service threads (of persistent connections)
- Add support for TCP Fast Open (require Linux kernel >= 3.16)
- Support HTTP POST method for graceful processing
- Double message buffer for more efficient processing of lengthy requests
- Correct/enhance ring buffer handling in certificate generations