MEDIUM: connection: automatically disable polling on error

MEDIUM: connection: automatically disable polling on error

We absolutely want to disable FD polling after an error is detected,
otherwise the data layer has to do it and it's far from being obvious
at these layers.

The way we did it was a bit tricky in conn_update_*_polling and
conn_*_polling_changes. However it has almost no impact on performance
and code size both for the fast and slow path.

We'll now be able to remove some flag updates in the stream interface.