Unset `Channel::is_usable` if mon update is blocking funding_locked

Unset `Channel::is_usable` if mon update is blocking funding_locked

If we have not yet sent `funding_locked` only because of a pending
channel monitor update, we shouldn't consider a channel
`is_usable`. This has a number of downstream effects, including
not attempting to route payments through the channel, not sending
private `channel_update` messages to our counterparty, or sending
channel_announcement messages if our couterparty has already signed
for it.

We further gate generation of `node_announcement`s on `is_usable`,
preventing generation of those or `announcement_signatures` until
we've sent our `funding_locked`.

Finally, `during_funding_monitor_fail` is updated to test a case
where we see the funding transaction lock in but have a pending
monitor update failure, then receive `funding_locked` from our
counterparty and ensure we don't generate the above messages until
after the monitor update completes.