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

doc/ostree-sections.txt

@@ -109,8 +109,7 @@ ostree_repo_stage_mtree
 ostree_repo_stage_commit
 OstreeRepoCheckoutMode
 OstreeRepoCheckoutOverwriteMode
-ostree_repo_checkout_tree_async
-ostree_repo_checkout_tree_finish
+ostree_repo_checkout_tree
 ostree_repo_checkout_gc
 ostree_repo_read_commit
 OstreeRepoListObjectsFlags