Move tracking of in-flight monitor updates to `ChannelManager`

[TheBlueMatt]

Now that the in-flight monitor updates are tracked in ChannelManager anyway, the monitor updated event from ChainMonitor should indicate the specific monitor update, rather than a total completion. Worse, we need to do something about having multiple max_value updates in-flight at once, figuring out which is the one that completed.

[TheBlueMatt]

This is still required for 100% complete async support, but at this point it looks like async support in 0.0.117 is going to have to live with being 90% complete, so this will slip to 0.0.118.