In this commit, we modify the sub-swapper to update the set of backups
files time it's tarted. We do this to ensure that each time we start,
we're up to sync with the current set of open channels. This also ensure
that we'll create a new back up file if this is the first time we're
starting up with this new feature.
In this commit, we introduce the chanbackup.SubSwapper interface. It
takes a regular Swapper implementation (defined by the
chanbackup.SubSwapper) interface along with a chanbackup.ChannelNotifier
implementation. Given these two interfaces, we're able to be notified
when a new channel is opened or closed, and then use the Swapper to
atomically replace the on-disk channel back up. As a result, a Lightning
daemon can ensure that they alwayts have a up to date channels.backup on
disk that can safely be copied away by users and be used to restore
channel funds in the event of partial/total data loss.
