[BUG] stream_sock: check for shut{r,w} before refreshing some timeouts

[BUG] stream_sock: check for shut{r,w} before refreshing some timeouts

Under some circumstances, it appears possible to refresh a timeout
just after a side has been shut. For instance, if poll() plans to
call both read and write, and the read side calls chk_snd() which
in turn causes a shutw to occur, then stream_sock_write could update
its write timeout. The same problem happens the other way.

The timeout checks will then not catch these cases because they
ignore timeouts in case of shut{r,w}.

This is very likely to be the major cause of the 100% CPU usages
reported by Bart Bobrowski.

The fix consists in always ensuring that a side is not shut before
updating its timeout.