Add types for updating ChannelMonitors without copying them.

Add types for updating ChannelMonitors without copying them.

This is the first step in migrating ChannelMonitor updating logic
to use incremental Update objects instead of copying the
ChannelMonitors themselves and insert_combine()ing them.

This adds most of the scaffolding and updates relevant comments to
refer to the new architecture, without changing how any actual
updates occur.