Add fastcgi_finish_request where appropriate

When using PHP-FPM to run PHP on a web server, the function
fastcgi_finish_request is provided, which flushes all response
data to the client and closes the connection, allowing PHP
to perform additional work without delaying the client.

This adds fastcgi_finish_request calls in index.php and api.php
where appropriate and if the function exists, so that the connection
is closed once the output is definitely finished.

Change-Id: Ic67a8956545874e94df5198088c0f4aa58ca376e