Fixes T3857. Earlier work made this trivial and just left product questions, which I've answered by requiring the daemons to run on reasonable installs.

Ran bin/search index and bin/search index --background. Observed indexes write in the former case and tasks queue in the latter case. Commented with a unique string on a revision and searched for it a moment later, got exactly one result (that revision), verifying that reindexing works correctly.