Fix timed schedulers (WIP)

Hi!

I'm just (sloooowly) trying out nine and the first thing I noticed is that the timed schedulers stopped working, caused by the changes to the scheduler API (`addBuildsetForLatest` removal, #597).

This probably went unnoticed because the scheduler's unit tests simulate that part of the API to spy on the method calls.

I already "fixed" the tests (update them to use `addBuildsetForSourceStampsWithDefaults`) and replaced some calls in the Nightly- and Periodic- schedulers.

However, the schedulers take a `branch` parameter and used that with `addBuildsetForLatest`. I'm not really sure how to handle that. Looks like it should use a codebase instead, but the details are a bit fuzzy…
I was thinking about using `Timed.startBuild` in `NightlyTriggerable.startBuild`, but I ended up using `addBuildset…` because I could not figure out how to attach the collected changes to the sourcestamps from the trigger. I'll remove the `**kwargs`.