repo: Move the transaction stats to a separate struct

This is much easier for callers to handle, and simplifies
the API a lot.

https://bugzilla.gnome.org/show_bug.cgi?id=707644

src/libostree/ostree-repo-private.h

@@ -40,11 +40,7 @@ struct OstreeRepo {
 
   GFile *transaction_lock_path;
   GMutex txn_stats_lock;
-  guint txn_metadata_objects_total;
-  guint txn_metadata_objects_written;
-  guint txn_content_objects_total;
-  guint txn_content_objects_written;
-  guint64 txn_content_bytes_written;
+  OstreeRepoTransactionStats txn_stats;
 
   GMutex cache_lock;
   GPtrArray *cached_meta_indexes;