See http://github.com/facebook/phabricator/issues/487. By default, we perform a write in this query to moved daemons to "dead" status after a timeout. This is normally reasonable, but after D7964 we do a setup check against the daemons, which means this query is invoked very early in the stack, before we have a write guard.
Since doing this write unconditionally is unnecessarily, surprising, and overly ambitious, make the write conditional and do not attempt to perform it from the setup check.
(We could also move this to a GC/cron sort of thing eventually, maybe -- it's a bit awkward here, but we don't have other infrastructure which is a great fit right now.)