BUG/MEDIUM: stconn/stream: Forward shutdown on write timeout

BUG/MEDIUM: stconn/stream: Forward shutdown on write timeout

The commit 7f59d68fe2 ("BUG/MEDIIM: stconn: Flush output data before
forwarding close to write side") introduced a regression. When a write
timeout is detected, the shutdown is no longer forwarded. Dependig on the
channels state, it may block the processing, waiting the client or the
server leaves.

The commit above tries to avoid to truncate messages on shutdown but on
write timeout, if the channel is not empty, there is nothing more we can do
to send these data. It means the endpoint is unable to send data. In this
case, we must forward the shutdown.

This patch should be backported as far as 2.2.