Requiring a migration w/ a source plugin using a generator fatals

Problem/Motivation

1. Create a migration where initializeIterator yields. This is a great fit and works well.
2. Now add this as a requirement to another migration.
3. SourcePluginBase::count() gets called which in turn does $count = $this->getIterator()->count(); which fatals as Generator does not implement count().
4. See $this->skipCount at the top of the method, add protected $skipCount = TRUE to the class.
5. Observe the constructor unconditionally overriding this value.
6. Tear your hair out.

Proposed resolution

Uh, I dunno, at least check whether the iterator is Countable before trying to count the bloody thing. And/or only pick up the skip_count from the configuration if it is set. This is one of those D7 vestiges which got ported and abandoned to bite us later.

Remaining tasks

doCount could use a little doxygen.

Write a test source plugin which uses yield and try to count it and assert it is -1. Check the configuration keys are picked up properly.

User interface changes

API changes

Data model changes