lib/repo: Add MT support for transaction_set_ref(), clarify MT rules

For rpm-ostree I'd like to do importing in parallel with threads; the code is
*almost* ready for that except today it calls
`ostree_repo_transaction_set_ref()`.

Looking at the code, there's really a "transaction" struct here,
not just stats.  Let's lift that struct out, and move the refs
into it under the existing lock.

Clarify the documentation around multithreading for various functions.

Closes: #1358
Approved by: jlebon

src/libostree/ostree-repo.c

@@ -505,13 +505,13 @@ ostree_repo_finalize (GObject *object)
     g_hash_table_destroy (self->updated_uncompressed_dirs);
   if (self->config)
     g_key_file_free (self->config);
-  g_clear_pointer (&self->txn_refs, g_hash_table_destroy);
-  g_clear_pointer (&self->txn_collection_refs, g_hash_table_destroy);
+  g_clear_pointer (&self->txn.refs, g_hash_table_destroy);
+  g_clear_pointer (&self->txn.collection_refs, g_hash_table_destroy);
   g_clear_error (&self->writable_error);
   g_clear_pointer (&self->object_sizes, (GDestroyNotify) g_hash_table_unref);
   g_clear_pointer (&self->dirmeta_cache, (GDestroyNotify) g_hash_table_unref);
   g_mutex_clear (&self->cache_lock);
-  g_mutex_clear (&self->txn_stats_lock);
+  g_mutex_clear (&self->txn_lock);
   g_free (self->collection_id);
 
   g_clear_pointer (&self->remotes, g_hash_table_destroy);
@@ -672,7 +672,7 @@ ostree_repo_init (OstreeRepo *self)
                                                  test_error_keys, G_N_ELEMENTS (test_error_keys));
 
   g_mutex_init (&self->cache_lock);
-  g_mutex_init (&self->txn_stats_lock);
+  g_mutex_init (&self->txn_lock);
 
   self->remotes = g_hash_table_new_full (g_str_hash, g_str_equal,
                                          (GDestroyNotify) NULL,