Rm ChannelMonitor merge capabilities in favor of explicit add/update

This removes the ability to merge ChannelMonitors in favor of
explicit ChannelMonitorUpdates. It further removes
ChannelManager::test_restore_channel_monitor in favor of the new
ChannelManager::channel_monitor_updated method, which explicitly
confirms a set of updates instead of providing the latest copy of
each ChannelMonitor to the user.

This removes almost all need for Channels to have the latest
channel_monitor, except for broadcasting the latest local state.