Get rid of unnecessary ChannelMonitor in Channel

[valentinewallace]

In preparation for refactoring how we persist channel data, we need to get rid of the Channel's ChannelMonitor. The only reason it exists now is because of the ChannelManager calling would_broadcast_at_height. would_broadcast_at_height provides information that should be accessible in a different way (that doesn't require the Channel to hold its ChannelMonitor).