channeldb: use kvdb.Backend instead of channeldb.DB for the Graph

channeldb/db.go

@@ -269,7 +269,7 @@ func CreateWithBackend(backend kvdb.Backend, modifiers ...OptionModifier) (*DB,
 		dryRun:  opts.dryRun,
 	}
 	chanDB.graph = newChannelGraph(
-		chanDB, opts.RejectCacheSize, opts.ChannelCacheSize,
+		backend, opts.RejectCacheSize, opts.ChannelCacheSize,
 		opts.BatchCommitInterval,
 	)