Use an opaque type to describe monitor updates in Persist

Use an opaque type to describe monitor updates in Persist

In the next commit, we'll be originating monitor updates both from
the ChainMonitor and from the ChannelManager, making simple
sequential update IDs impossible.

Further, the existing async monitor update API was somewhat hard to
work with - instead of being able to generate monitor_updated
callbacks whenever a persistence process finishes, you had to
ensure you only did so at least once all previous updates had also
been persisted.

Here we eat the complexity for the user by moving to an opaque
type for monitor updates, tracking which updates are in-flight for
the user and only generating monitor-persisted events once all
pending updates have been committed.