Before this commit, the `(channeldb.DB).AddrsForNode` method treats the
results from the channel db and the graph db slightly differently. It
errors out if the channel db is unaware of the node in question but does
not error out if the graph is unaware of the node. So this commit
changes the logic slightly so that a "node-unknown" error from either
backing source is not seen as an error.
In this commit, we implement a version of the AddrSource interface which
merges the results of a set of AddrSource implementations. This will
later be used to merge the results of the channel db and graph db.
Our aim is to completely remove the `channeldb.DB`'s direct dependence
on the `graphdb.ChannelGraph` pointer. The only place where it still
depends on this pointer is in the `(DB).AddrsForNode(..)` method where
it queries both the channel DB and the graph db for the known addresses
for the node in question and then combines the results. So, to separate
these out, we will define an AddrsForNodes interface in this commit
which we will later let both the ChannelGraph and channeldb.DB both
implement and we will merge these results outside of the channeldb
package.
All this commit does is to unify the `AddrSource` interface since this
has been defined separately in a couple of places.
