When the rebuild backend is dropped, we must also drain the async channel.
This covers a corner case where a message may be sent at the same time as
we're dropping and in this case the message would hang.

This is not a hang for prod as there we have timeouts which would
eventually cancel the future and allow the drop, though this can still
lead to timeouts and confusion.

Signed-off-by: Tiago Castro <[email protected]>

This seems to have been mistakenly added as ms.
In practice this would have caused no harm as this value is not
currently being overrided by the helm chart.

Signed-off-by: Tiago Castro <[email protected]>

When we are pausing the nexus, all IO must get flushed before
the subsystem pausing completes.
If we can't flush the IO then pausing is stuck forever...

The issue we have seen is that when IO's are stuck there's
nothing which can fail them and allow pause to complete.
One way this can happen is when the controller is failed as
it seems in this case the io queues are not getting polled.

A first fix that can be done is to piggy back on the adminq
polling failure and use this to drive the removal of the
failed child devices from the nexus per-core channels.

A better approach might be needed in the future to be able
to timeout the IOs even when no completions are processed
in a given I/O qpair.

Signed-off-by: Tiago Castro <[email protected]>