Add API to retrieve statistics from transactions, use it in commit

It's just interesting to see =) This is also kind of prototyping out
some more "structured" output.

src/libostree/ostree-repo-private.h

@@ -38,6 +38,12 @@ struct OstreeRepo {
   GFile *config_file;
 
   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;
 
   GMutex cache_lock;
   GPtrArray *cached_meta_indexes;