Stop persisting background shutdown monitor updates

Stop persisting background shutdown monitor updates

In d4810087c1 we added logic to apply `ChannelMonitorUpdate`s which
were a part of a channel closure async via a background queue to
address some startup issues. When we did that we persisted those
updates to ensure we replayed them when starting next time.

However, there was no reason to - if we persisted and then
restarted even without those monitor updates we'd find a monitor
without a channel, which we'd tell to broadcast the latest
commitment transaction to force-close.

Since adding that logic, we've used the same background queue for
several purposes.