make Trigger Daemon sleep correctly when one-time triggers exist

Summary:
Trigger daemon is trying to find the next event to invoke before sleeping, but the query includes already-elapsed triggers.
It then tries to sleep for 0 seconds.

Test Plan:
On a new instance, schedule a single trigger of type PhabricatorOneTimeTriggerClock to a very near time.

Use top to see trigger daemon not going to 100% CPU once the event has elapsed.

Reviewers: #blessed_reviewers, epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D15750