In d1c340a0e1 we added support in
`handle_new_monitor_update!` for handling updates without dropping
locks.
In the coming commits we'll start handling `ChannelMonitorUpdate`s
"like normal" for updates against closed channels. Here we set up
the first step by adding a new `POST_CHANNEL_CLOSE` variant on
`handle_new_monitor_update!` which attempts to handle the
`ChannelMonitorUpdate` and handles completion actions if it
finishes immediately, just like the pre-close variant.