Web response optimization

- If fastcgi_finish_request is available then response will be sent and the rest of processing will happen in background.
- Else it will just flush response as soon as possible.
No because then the possibility for the race condition will be much higher i.e. next request will try using session while current one is still writing it. If it's not an issue somehow then this line could be removed altogether since we're closing session at shutdown anyway.