In this commit, we add the channelNotifier, and implementation fo the
chanbackup.ChannelNotifier interface. This will be our bridge from the
channelnotifier.ChannelNotifier sturct within lnd, and the interface
abstraction that the chanbackup.SubSwapper accpets. The role of this new
struct is simple: proxy the messages from the
channelnotifier.ChannelNotifier to the chanbackup.SubSwapper in a format
that it understands.
Along the way we introduce a tiny interface such that we don't need to
depend on the the channledb package.
