R1:19be32656f84

Implement clock/trigger infrastructure for scheduling actions

Summary:
Ref T6881. Hopefully, this is the hard part.

This adds a new daemon (the "trigger" daemon) which processes triggers, schedules them, and then executes them at the scheduled time. The design is a little complicated, but has these goals:

- High resistance to race conditions: only the application writes to the trigger table; only the daemon writes to the event table. We won't lose events if someone saves a meeting at the same time as…