Perform search indexing in the worker queue and respect `bin/search index --background`

Summary: 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.

Test Plan: 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.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3857

Differential Revision: https://secure.phabricator.com/D7966