Ensure `ChannelMonitorUpdate`s are ordered with full monitor writes

Ensure `ChannelMonitorUpdate`s are ordered with full monitor writes

When we update a channel, then while connecting a block persist a
full `ChannelMonitor` prior to persisting the
`ChannelMonitorUpdate`, users can end up seeing a full
`ChannelMonitor` with a given `latest_update_id` prior to seeing
the `ChannelMonitorUpdate` with the same `update_id`. This
could cause users to have a full `ChannelMonitor` on disk as well
as a `ChannelMonitorUpdate` which was already applied. While this
isn't an issue for the LDK-provided update-based `Persist`, its
somewhat surprising for users so we avoid it.