Add waiters to events

[kegsay]

We currently need to do things like

complement-crypto/tests/to_device_test.go

Line 186 in f871227

time.Sleep(time.Second)

because we don't listen for changes to the state of an event. This is flakey and has caused intermittent failures in this test and likely others.

Instead, we should hook into the timeline update code and every time we see our target event ID we should emit an update on a channel for it. That way, we can use the waiter pattern of "wait until this happens and give up after N seconds" which is far more reliable as N can be large without affecting the runtime of the test in the happy case.