fix - flushing temporary files after each worker request

[AlliBalliBaba]

Should close #1320.

After some trials with file upload I realized that this code is also a bit leaky (noticed after 100.000+ requests without restart):

while (frankenphp_handle_request(function () {
    echo file_get_contents('php://input');
})) {}

Seems like something relating to the input stream global is not properly cleared.