channeldb: define a single AddrSource interface

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.

server.go

@@ -263,7 +263,7 @@ type server struct {
 
 	chanStateDB *channeldb.ChannelStateDB
 
-	addrSource chanbackup.AddressSource
+	addrSource channeldb.AddrSource
 
 	// miscDB is the DB that contains all "other" databases within the main
 	// channel DB that haven't been separated out yet.