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/legacy_serialization.go

@@ -34,7 +34,7 @@ func deserializeCloseChannelSummaryV6(r io.Reader) (*ChannelCloseSummary, error)
 		return nil, err
 	}
 
-	if err := readChanConfig(r, &c.LocalChanConfig); err != nil {
+	if err := ReadChanConfig(r, &c.LocalChanConfig); err != nil {
 		return nil, err
 	}