channeldb/migration21: add new serialization to package 'current'

We copy the new serialization togic to a new package 'current' and
export the methods we will need from the migration. This ensures that we
have isolation between the legacy and current serialization methods,
cleanly inidcating what type we are using during the migration.

channeldb/migration21/legacy/legacy_decoding.go

@@ -556,7 +556,7 @@ func (p *ChannelPackager) LoadFwdPkgs(tx kvdb.RTx) ([]*common.FwdPkg, error) {
 }
 
 // loadChannelFwdPkgs loads all forwarding packages owned by `source`.
-func loadChannelFwdPkgs(tx kvdb.RTx, source lnwire.ShortChannelID) ([]*common.FwdPkg, error) {
+func loadChannelFwdPkgs(tx kvdb.RTx, source lnwire.ShortChannelID) ([]*common.FwdPkg, error) { // nolint: dupl
 	fwdPkgBkt := tx.ReadBucket(fwdPackagesKey)
 	if fwdPkgBkt == nil {
 		return nil, nil