Threads: fixed request hang with aio_write and subrequests.

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.