libostree: Check out directories depth-first in serial, switch to sync API

The way we recurse into subdirectories in parallel makes it far too
easy to hit up against the arbitrary Linux fd limit of 1024.

Since the fix here is about dropping parallelism, let's just go all
the way for now and make a plain old synchronous API =(

This does simplify both internal callers which wanted a sync API
anyways.

https://bugzilla.gnome.org/show_bug.cgi?id=706380

src/libostree/ostree-repo.h

@@ -326,21 +326,15 @@ typedef enum {
   OSTREE_REPO_CHECKOUT_OVERWRITE_UNION_FILES = 1
 } OstreeRepoCheckoutOverwriteMode;
 
-void
-ostree_repo_checkout_tree_async (OstreeRepo               *self,
-                                 OstreeRepoCheckoutMode    mode,
-                                 OstreeRepoCheckoutOverwriteMode    overwrite_mode,
-                                 GFile                    *destination,
-                                 OstreeRepoFile           *source,
-                                 GFileInfo                *source_info,
-                                 GCancellable             *cancellable,
-                                 GAsyncReadyCallback       callback,
-                                 gpointer                  user_data);
-
 gboolean
-ostree_repo_checkout_tree_finish (OstreeRepo               *self,
-                                  GAsyncResult             *result,
-                                  GError                  **error);
+ostree_repo_checkout_tree (OstreeRepo               *self,
+                           OstreeRepoCheckoutMode    mode,
+                           OstreeRepoCheckoutOverwriteMode    overwrite_mode,
+                           GFile                    *destination,
+                           OstreeRepoFile           *source,
+                           GFileInfo                *source_info,
+                           GCancellable             *cancellable,
+                           GError                  **error);
 
 gboolean       ostree_repo_checkout_gc (OstreeRepo        *self,
                                         GCancellable      *cancellable,