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…
Correct me if I'm wrong with the logic here: the conditional above only applies when `lastTrigger` is in the format used by the current master branch -- that is, never in released code.  So we never need to worry about finding `lastTrigger` as a 2-tuple with a list as its first element, and the conditional below ("handle state from before Buildbot-0.9.0") takes care of all conversions from old `lastTrigger` formats to new the new 4-tuple format.