Threads: fixed request hang with aio_write and subrequests. If the subrequest is already finalized, the handler set with aio_write may still be used by sendfile in threads when using range requests (see also e4c1f5b32868, and the original note in 9fd738b85fad). Calling already finalized subrequest's r->write_event_handler in practice results in request hang in some cases. Fix is to trigger connection event handler if the subrequest was already finalized.