Fix: only flush temporary unreferenced streams

[AlliBalliBaba]

This PR refines the fix in #1350.

I'll do some more testing, but explicitly checking if the stream is referenced as a zval will prevent issues with curl/openssl on a Laravel application I've been testing with.

[withinboredom]

This makes a lot of sense, I think. If I understand correctly, we basically only delete temp streams? Do we need to consider pure memory streams as well, or does this cover it?

[AlliBalliBaba]

This should cover all temporary streams created via php_stream_temp_create_ex. The important part is checking for stream->__exposed.

Tests actually also pass if all we do is check for stream->__exposed == 0. So any stream that is unreachable by PHP:

if (val->type == stream_type) {
  php_stream *stream = (php_stream *)val->ptr;
  if (stream->__exposed == 0) {
    zend_list_close(val);
    zend_list_delete(val);
  }
}

The other checks are only there for safety and might not be necessary.

[AlliBalliBaba]

In other words, any stream reachable by PHP is also reachable by GC. We want to clean up the streams not reachable by the GC.