channeldb: slightly refactor the [Fetch|Put]Meta methods

This commit performs a slight refactoring of the internals (and API) of
the [Fetch|Put]Meta methods. The changes are rather minor and simply
eliminate the conditional branching structure with usage of an internal
function. This new form is much easier to follow.

channeldb/db.go

@@ -337,7 +337,7 @@ func (d *DB) syncVersions(versions []version) error {
 		}
 
 		meta.DbVersionNumber = latestVersion
-		if err := d.PutMeta(meta, tx); err != nil {
+		if err := putMeta(meta, tx); err != nil {
 			return err
 		}