Handle `BackgroundEvent`s replaying non-closing monitor updates

Handle `BackgroundEvent`s replaying non-closing monitor updates

`BackgroundEvent` was used to store `ChannelMonitorUpdate`s which
result in a channel force-close, avoiding relying on
`ChannelMonitor`s having been loaded while `ChannelManager`
block-connection methods are called during startup.

In the coming commit(s) we'll also generate non-channel-closing
`ChannelMonitorUpdate`s during startup, which will need to be
replayed prior to any other `ChannelMonitorUpdate`s generated from
normal operation.

In the next commit we'll handle that by handling `BackgroundEvent`s
immediately after locking the `total_consistency_lock`.