DirMgr: Stop load-from-cache process when there is no change.

Previously in !511 (merged) I had introduced a bug where, if there was an error more serious than "no change", that error would keep us from noticing that we had no change, and we'd loop until the safety counter ran out. Then we'd panic.

This commit fixes the bug by calling add_from_cache() one document at a time.

Fixes #482 (closed).