Set `channel_update` disable bit based on staged even for onions

Set `channel_update` disable bit based on staged even for onions

When generating a `channel_update` either in response to a fee
configuration change or an HTLC failure, we currently poll the
channel to check if the peer's connected when setting the disabled
bit in the `channel_update`. This could cause cases where we set
the disable bit even though the peer *just* disconnected, and don't
generate a followup broadcast `channel_update` with the disabled
bit unset.

While a node generally shouldn't rebroadcast a `channel_update` it
received in an onion, there's nothing inherently stopping them from
doing so. Obviously in the fee-update case we expect the message to
propagate.

Luckily, since we already "stage" disable-changed updates, we can
check the staged state and use that to set the disabled bit in all
`channel_update` cases.