sched: Ignore closed channel after flushing cells

sched: Ignore closed channel after flushing cells

The flush cells process can close a channel if the connection write fails but
still return that it flushed at least one cell. This is due because the error
is not propagated up the call stack so there is no way of knowing if the flush
actually was successful or not.

Because this would require an important refactoring touching multiple
subsystems, this patch is a bandaid to avoid the KIST scheduler to handle
closed channel in its loop.

Bandaid on #23751.

Signed-off-by: David Goulet <dgoulet@torproject.org>