When importing several sources in parallel via http(s) mboxrd,
we need to be able to get keywords of uncommitted documents
directly from shard workers. Otherwise, Xapian DocNotFound
errors happen because the read-only LeiSearch won't see
documents from uncomitted transactions. Keep in mind that it's
possible the keywords can be changed on-the-fly even for
uncommitted documents because of inotify watches from LeiNoteEvent.