Move ChannelManager::monitor_updated to a MonitorEvent

Move ChannelManager::monitor_updated to a MonitorEvent

In the next commit we'll need ChainMonitor to "see" when a monitor
persistence completes, which means `monitor_updated` needs to move
to `ChainMonitor`. The simplest way to then communicate that
information to `ChannelManager` is via `MonitorEvet`s, which seems
to line up ok, even if they're now constructed by multiple
different places.