implement fastcgi_finish_request()

[wwbmmm]

Change-Id: I6a8322e7030060c619397cd39a94c660c1f214ce

This resolve #1230.

fastcgi_finish_request() is very useful to send response instantly and do the rest thing in background.

register_postsend_function() can do the same thing, but it is not compatible with Zend.