lib/repo: Add locking auto cleanup handler

Define an auto cleanup handler for use with repo locking. This is based
on the existing auto transaction cleanup. A wrapper for
ostree_repo_lock_push() is added with it. The intended usage is like so:

  g_autoptr(OstreeRepoAutoLock) lock = NULL;
  lock = ostree_repo_auto_lock_push (repo, lock_type, cancellable, error);
  if (!lock)
    return FALSE;

The functions and type are marked to be skipped by introspection since I
can't see them being usable from bindings.

Closes: #1343
Approved by: cgwalters

apidoc/ostree-experimental-sections.txt

@@ -93,6 +93,9 @@ ostree_repo_finder_override_get_type
 OstreeRepoLockType
 ostree_repo_lock_push
 ostree_repo_lock_pop
+OstreeRepoAutoLock
+ostree_repo_auto_lock_push
+ostree_repo_auto_lock_cleanup
 ostree_repo_get_collection_id
 ostree_repo_set_collection_id
 ostree_validate_collection_id