channeldb+migration: export commonly used methods

This commit exports several commonly used methods that can be used by
later migrations. It also adds a channel commit deserializer.

channeldb/migration_01_to_11/graph.go

@@ -836,7 +836,7 @@ func deserializeChanEdgeInfo(r io.Reader) (ChannelEdgeInfo, error) {
 	}
 
 	edgeInfo.ChannelPoint = wire.OutPoint{}
-	if err := readOutpoint(r, &edgeInfo.ChannelPoint); err != nil {
+	if err := ReadOutpoint(r, &edgeInfo.ChannelPoint); err != nil {
 		return ChannelEdgeInfo{}, err
 	}
 	if err := binary.Read(r, byteOrder, &edgeInfo.Capacity); err != nil {