Add tests for AbsoluteSourceStampsMixin

Also, remove 3 tests from test_schedulers_basic. The desired
functionality is implemented and tested in the mixin.
Move AbsoluteSourceStampMixin up the inheritance chain
Add a method to start a build with collected changes

This can be used by the startBuild implementations. If the scheduler
has collected and classified any changes, it will use
`addBuildsetForChanges` and flush the change classifications. If there
are no changes it will use `addBuildsetForSourceStampsWithDefaults`.
If `onlyIfChanged` was given and there are no important changes, the
method will return without triggering any builds. Collected changes
won't be flushed from the database.
Fix onlyIfChanged, adjust Periodic
Fix NightlyTriggerable
Move onlyIfChanged to Timed scheduler base class.
Wrap some overly long lines
Allow createAbsoluteSourceStamps without onlyIfChanged
Let's not redefine the builtin filter(), mkay?
Start consuming changes in Timed (instead of Nightly)
Update manual and relnotes for timed scheduler changes.
Fix typo(?) in scheduler helper

The method is called addBuildsetForSourceStamps (plural) not
addBuildsetForSourceStamp.