BUG/MEDIUM: stconn: Update stream expiration date on blocked sends

BUG/MEDIUM: stconn: Update stream expiration date on blocked sends

When outgoing data are blocked, we must update the stream expiration date
and requeue the task. It is important to be sure to properly handle write
timeout, expecially if the stream cannot expire on reads. This bug was
introduced when handling of channel's timeouts was refactored to be managed
by the stream-connectors.

It is an issue if there is no server timeout and the client does not consume
the response (or the opposite but it is less common). It is also possible to
trigger the same scenario with applets on server side because, most of time,
there is no server timeout.

This patch must be backported to 2.8.