Run monitor update completion actions for pre-startup completion

Run monitor update completion actions for pre-startup completion

If a `ChannelMonitorUpdate` completes being persisted, but the
`ChannelManager` isn't informed thereof (or isn't persisted) before
shutdown, on startup we may still have it listed as in-flight. When
we compare the available `ChannelMonitor` with the in-flight set,
we'll notice it completed and remove it, however this may leave
some post-update actions dangling which need to complete.

Here we handle this with a new `BackgroundEvent` indicating we need
to handle any post-update action(s) for a given channel.